Japa: This is just a mock-up, but here you go:
the new "_MDF_decorations_3.png" for data/art. drag & drop & replace old one.
the new lines for the override.txt in data/init. Add at the bottom.
##################################################################################
#different wall tiles for wood
OVERRIDE:43:T:ConstructedFloor:_MDF_overrides_6:221
OVERRIDE:206:T:ConstructedFortification:_MDF_overrides_6:184
OVERRIDE:62:T:ConstructedStairD:_MDF_overrides_6:186
OVERRIDE:60:T:ConstructedStairU:_MDF_overrides_6:187
OVERRIDE:88:T:ConstructedStairUD:_MDF_overrides_6:185
[OVERRIDE:199:T:ConstructedPillar:_MDF_overrides_6:126:::WOOD]
[OVERRIDE:186:T:ConstructedWallUD:_MDF_overrides_6:123:::WOOD]
[OVERRIDE:205:T:ConstructedWallLR:_MDF_overrides_6:142:::WOOD]
[OVERRIDE:200:T:ConstructedWallRU:_MDF_overrides_6:127:::WOOD]
[OVERRIDE:211:T:ConstructedWallRU:_MDF_overrides_6:127:::WOOD]
[OVERRIDE:212:T:ConstructedWallRU:_MDF_overrides_6:127:::WOOD]
[OVERRIDE:200:T:ConstructedWallRU2:_MDF_overrides_6:127:::WOOD]
[OVERRIDE:211:T:ConstructedWallRU2:_MDF_overrides_6:127:::WOOD]
[OVERRIDE:212:T:ConstructedWallRU2:_MDF_overrides_6:127:::WOOD]
[OVERRIDE:200:T:ConstructedWallR2U:_MDF_overrides_6:127:::WOOD]
[OVERRIDE:211:T:ConstructedWallR2U:_MDF_overrides_6:127:::WOOD]
[OVERRIDE:212:T:ConstructedWallR2U:_MDF_overrides_6:127:::WOOD]
[OVERRIDE:188:T:ConstructedWallLU:_MDF_overrides_6:125:::WOOD]
[OVERRIDE:189:T:ConstructedWallLU:_MDF_overrides_6:125:::WOOD]
[OVERRIDE:190:T:ConstructedWallLU:_MDF_overrides_6:125:::WOOD]
[OVERRIDE:188:T:ConstructedWallLU2:_MDF_overrides_6:125:::WOOD]
[OVERRIDE:189:T:ConstructedWallLU2:_MDF_overrides_6:125:::WOOD]
[OVERRIDE:190:T:ConstructedWallLU2:_MDF_overrides_6:125:::WOOD]
[OVERRIDE:188:T:ConstructedWallL2U:_MDF_overrides_6:125:::WOOD]
[OVERRIDE:189:T:ConstructedWallL2U:_MDF_overrides_6:125:::WOOD]
[OVERRIDE:190:T:ConstructedWallL2U:_MDF_overrides_6:125:::WOOD]
[OVERRIDE:213:T:ConstructedWallRD:_MDF_overrides_6:138:::WOOD]
[OVERRIDE:214:T:ConstructedWallRD:_MDF_overrides_6:138:::WOOD]
[OVERRIDE:201:T:ConstructedWallRD:_MDF_overrides_6:138:::WOOD]
[OVERRIDE:213:T:ConstructedWallRD2:_MDF_overrides_6:138:::WOOD]
[OVERRIDE:214:T:ConstructedWallRD2:_MDF_overrides_6:138:::WOOD]
[OVERRIDE:201:T:ConstructedWallRD2:_MDF_overrides_6:138:::WOOD]
[OVERRIDE:213:T:ConstructedWallR2D:_MDF_overrides_6:138:::WOOD]
[OVERRIDE:214:T:ConstructedWallR2D:_MDF_overrides_6:138:::WOOD]
[OVERRIDE:201:T:ConstructedWallR2D:_MDF_overrides_6:138:::WOOD]
[OVERRIDE:183:T:ConstructedWallLD:_MDF_overrides_6:124:::WOOD]
[OVERRIDE:184:T:ConstructedWallLD:_MDF_overrides_6:124:::WOOD]
[OVERRIDE:187:T:ConstructedWallLD:_MDF_overrides_6:124:::WOOD]
[OVERRIDE:183:T:ConstructedWallLD2:_MDF_overrides_6:124:::WOOD]
[OVERRIDE:184:T:ConstructedWallLD2:_MDF_overrides_6:124:::WOOD]
[OVERRIDE:187:T:ConstructedWallLD2:_MDF_overrides_6:124:::WOOD]
[OVERRIDE:183:T:ConstructedWallL2D:_MDF_overrides_6:124:::WOOD]
[OVERRIDE:184:T:ConstructedWallL2D:_MDF_overrides_6:124:::WOOD]
[OVERRIDE:187:T:ConstructedWallL2D:_MDF_overrides_6:124:::WOOD]
[OVERRIDE:202:T:ConstructedWallLRU:_MDF_overrides_6:139:::WOOD]
[OVERRIDE:203:T:ConstructedWallLRD:_MDF_overrides_6:140:::WOOD]
[OVERRIDE:185:T:ConstructedWallLUD:_MDF_overrides_6:122:::WOOD]
[OVERRIDE:204:T:ConstructedWallRUD:_MDF_overrides_6:141:::WOOD]
[OVERRIDE:206:T:ConstructedWallLRUD:_MDF_overrides_6:143:::WOOD]
##################################################################################
#different wall tiles for metal
OVERRIDE:43:T:ConstructedFloor:_MDF_overrides_6:221
OVERRIDE:206:T:ConstructedFortification:_MDF_overrides_6:184
OVERRIDE:62:T:ConstructedStairD:_MDF_overrides_6:186
OVERRIDE:60:T:ConstructedStairU:_MDF_overrides_6:187
OVERRIDE:88:T:ConstructedStairUD:_MDF_overrides_6:185
[OVERRIDE:199:T:ConstructedPillar:_MDF_overrides_6:158:::IS_METAL]
[OVERRIDE:186:T:ConstructedWallUD:_MDF_overrides_6:155:::IS_METAL]
[OVERRIDE:205:T:ConstructedWallLR:_MDF_overrides_6:174:::IS_METAL]
[OVERRIDE:200:T:ConstructedWallRU:_MDF_overrides_6:159:::IS_METAL]
[OVERRIDE:211:T:ConstructedWallRU:_MDF_overrides_6:159:::IS_METAL]
[OVERRIDE:212:T:ConstructedWallRU:_MDF_overrides_6:159:::IS_METAL]
[OVERRIDE:200:T:ConstructedWallRU2:_MDF_overrides_6:159:::IS_METAL]
[OVERRIDE:211:T:ConstructedWallRU2:_MDF_overrides_6:159:::IS_METAL]
[OVERRIDE:212:T:ConstructedWallRU2:_MDF_overrides_6:159:::IS_METAL]
[OVERRIDE:200:T:ConstructedWallR2U:_MDF_overrides_6:159:::IS_METAL]
[OVERRIDE:211:T:ConstructedWallR2U:_MDF_overrides_6:159:::IS_METAL]
[OVERRIDE:212:T:ConstructedWallR2U:_MDF_overrides_6:159:::IS_METAL]
[OVERRIDE:188:T:ConstructedWallLU:_MDF_overrides_6:157:::IS_METAL]
[OVERRIDE:189:T:ConstructedWallLU:_MDF_overrides_6:157:::IS_METAL]
[OVERRIDE:190:T:ConstructedWallLU:_MDF_overrides_6:157:::IS_METAL]
[OVERRIDE:188:T:ConstructedWallLU2:_MDF_overrides_6:157:::IS_METAL]
[OVERRIDE:189:T:ConstructedWallLU2:_MDF_overrides_6:157:::IS_METAL]
[OVERRIDE:190:T:ConstructedWallLU2:_MDF_overrides_6:157:::IS_METAL]
[OVERRIDE:188:T:ConstructedWallL2U:_MDF_overrides_6:157:::IS_METAL]
[OVERRIDE:189:T:ConstructedWallL2U:_MDF_overrides_6:157:::IS_METAL]
[OVERRIDE:190:T:ConstructedWallL2U:_MDF_overrides_6:157:::IS_METAL]
[OVERRIDE:213:T:ConstructedWallRD:_MDF_overrides_6:170:::IS_METAL]
[OVERRIDE:214:T:ConstructedWallRD:_MDF_overrides_6:170:::IS_METAL]
[OVERRIDE:201:T:ConstructedWallRD:_MDF_overrides_6:170:::IS_METAL]
[OVERRIDE:213:T:ConstructedWallRD2:_MDF_overrides_6:170:::IS_METAL]
[OVERRIDE:214:T:ConstructedWallRD2:_MDF_overrides_6:170:::IS_METAL]
[OVERRIDE:201:T:ConstructedWallRD2:_MDF_overrides_6:170:::IS_METAL]
[OVERRIDE:213:T:ConstructedWallR2D:_MDF_overrides_6:170:::IS_METAL]
[OVERRIDE:214:T:ConstructedWallR2D:_MDF_overrides_6:170:::IS_METAL]
[OVERRIDE:201:T:ConstructedWallR2D:_MDF_overrides_6:170:::IS_METAL]
[OVERRIDE:183:T:ConstructedWallLD:_MDF_overrides_6:156:::IS_METAL]
[OVERRIDE:184:T:ConstructedWallLD:_MDF_overrides_6:156:::IS_METAL]
[OVERRIDE:187:T:ConstructedWallLD:_MDF_overrides_6:156:::IS_METAL]
[OVERRIDE:183:T:ConstructedWallLD2:_MDF_overrides_6:156:::IS_METAL]
[OVERRIDE:184:T:ConstructedWallLD2:_MDF_overrides_6:156:::IS_METAL]
[OVERRIDE:187:T:ConstructedWallLD2:_MDF_overrides_6:156:::IS_METAL]
[OVERRIDE:183:T:ConstructedWallL2D:_MDF_overrides_6:156:::IS_METAL]
[OVERRIDE:184:T:ConstructedWallL2D:_MDF_overrides_6:156:::IS_METAL]
[OVERRIDE:187:T:ConstructedWallL2D:_MDF_overrides_6:156:::IS_METAL]
[OVERRIDE:202:T:ConstructedWallLRU:_MDF_overrides_6:171:::IS_METAL]
[OVERRIDE:203:T:ConstructedWallLRD:_MDF_overrides_6:172:::IS_METAL]
[OVERRIDE:185:T:ConstructedWallLUD:_MDF_overrides_6:154:::IS_METAL]
[OVERRIDE:204:T:ConstructedWallRUD:_MDF_overrides_6:173:::IS_METAL]
[OVERRIDE:206:T:ConstructedWallLRUD:_MDF_overrides_6:175:::IS_METAL]
EDIT: The override.txt also has overrides for all normal walls without material mentioned, not sure if they clash with the new system. Just CTRL+F for "#different wall tiles" to find the entries. Maybe you can delete them for testing.
waterphage13 & Pvt. Pirate: True, some look a bit weird ingame, but it depends heavily on the color scheme you install.
jecowa: They are dragonmasters set; but I ported a few of them (10 or so) once to have a look.
EDIT: OH. I can do subtypes for encased-in-ice items. mh. That's good news on the one hand, but it also means that I'd have to add several hundred sprites just for that. ^^