AH HAH AND EUREKA!
I was looking at your smelting stuff and it hit me, add in a TOOL_BONE_CLEAN item, a reaction to produce them from a stack of bones 1 for 1 using a knife, and use the cleaned bones + a bone from the stack to make up the multiple-bone requirements.
Toss that in and suddenly the problem of not being able to break a stack of 80 bones into individual parts while still using more than 1 per reaction is quite approachable, and it even makes sense to clean and set aside a stack of bones which you're going to work on.
[ITEM_TOOL:ITEM_TOOL_BONE_CLEAN]
[NAME:cleaned bone:cleaned bones]
[VALUE:10]
[TILE:236]
[SIZE:300]
[MATERIAL_SIZE:1]
[UNIMPROVABLE]
------------
[REACTION:BONECARVING_ADV]
[NAME:separate bones]
[ADVENTURE_MODE_ENABLED]
[REAGENT:knife:1:TOOL:ITEM_TOOL_KNIFE:NONE:NONE][PRESERVE_REAGENT]
[REAGENT:bones:1:NONE:NONE:NONE:NONE][USE_BODY_COMPONENT][ANY_BONE_MATERIAL]
[PRODUCT:100:1:TOOL:ITEM_TOOL_BONE_CLEAN:GET_MATERIAL_FROM_REAGENT:bones:NONE]
[SKILL:BONECARVE]
[REACTION:CRAFT_BONE_BREASTPLATE_ADV]
[NAME:craft bone breastplate]
[ADVENTURE_MODE_ENABLED]
[REAGENT:knife:1:TOOL:ITEM_TOOL_KNIFE:NONE:NONE][PRESERVE_REAGENT]
[REAGENT:needle:1:TOOL:ITEM_TOOL_NEEDLE:NONE:NONE][PRESERVE_REAGENT]
[REAGENT:bones:1:NONE:NONE:NONE:NONE][USE_BODY_COMPONENT][ANY_BONE_MATERIAL]
[REAGENT:cleaned bones:4:TOOL:ITEM_TOOL_BONE_CLEAN:NONE:NONE]
[REAGENT:padding:1:SKIN_TANNED:NONE:NONE:NONE]
[PRODUCT:100:1:ARMOR:ITEM_ARMOR_BREASTPLATE:GET_MATERIAL_FROM_REAGENT:bones:NONE]
[SKILL:BONECARVE]
I managed to avoid a dragon long enough for it to bleed out after a lucky throat slash landed my first try, pulled the heart out and embalmed it, ate it, made a second for the combined balm if I can manage to track down the other beasts necessary and kill them, started hacking away at the corpse and was able to get myself suited up with mostly exceptional stuff, a couple of superior pieces, and I think two random masterworks before I ran out of parts, but it was definitely close, and the time I spent hacking away at a couple random trolls and dingos and such paid off.
I never felt really satisfied with the way a dragon skeleton turned into 200+ chances to roll a full set of masterwork gear, this though felt cool.