I believe what you currently have will ALMOST work; however, the dissolving body part must have ONLY the melting tissue, so your [TISSUE_LAYER:BY_CATEGORY:ALL:UNIFORM_TISSUE] token would cause it to not work. Also, I believe that if a creature's [LOWERBODY] part melts, it will die instantly.
Here is an example drawn from a working limb-dropping creature. The 'egg sack' body parts melt, releasing a large number of 'hatchling' body parts that can be raised by the parent creature. Note that the egg sack is a nonessential body part, and has a unique CATEGORY token so that it doesn't get tissues assigned to it by the BODY_DETAIL_PLAN.
This is just the bit of the parent creature that shows the body and the melting tissue:
[BODY:GOHMA_BODY:BOSS_EYE:SMALL_3PARTCLAWARMS:TAIL_OVI:HEART:GUTS:MOUTH:EGG_RACK_10_NEW]
[BODY_DETAIL_PLAN:CHITIN_MATERIALS]
[USE_MATERIAL_TEMPLATE:MELTING_EGG_SACK:MELTING_EGG_SACK_TEMPLATE]
[TISSUE:MELTING_EGG_SACK]
[TISSUE_NAME:egg sack:NP]
[TISSUE_MATERIAL:LOCAL_CREATURE_MAT:MELTING_EGG_SACK]
[TISSUE_SHAPE:LAYER]
[RELATIVE_THICKNESS:1]
[BODY_DETAIL_PLAN:MELTING_EGG_SACK_PLAN]
[BODY_DETAIL_PLAN:CHITIN_TISSUES]
[BODY_DETAIL_PLAN:EXOSKELETON_TISSUE_LAYERS:CHITIN:FAT:MUSCLE]
[BODY:EGG_RACK_10_NEW]
[BP:RACK1:egg sack:egg sacks][CON:UB][CATEGORY:EGG_RACK][SMALL]
[DEFAULT_RELSIZE:1]
[BP:EB_UB1:hatchling:hatchlings][CON:RACK1][CATEGORY:EGG][SMALL][GRASP][NUMBER:10][CATEGORY:BODY]
[DEFAULT_RELSIZE:1]
[MATERIAL_TEMPLATE:MELTING_EGG_SACK_TEMPLATE]
[STATE_COLOR:ALL_SOLID:CARMINE]
[STATE_NAME:ALL_SOLID:egg sack]
[STATE_ADJ:ALL_SOLID:egg sack]
[STATE_COLOR:LIQUID:CARMINE]
[STATE_NAME:LIQUID:burst egg sack]
[STATE_ADJ:LIQUID:egg sack]
[STATE_COLOR:GAS:CARMINE]
[STATE_NAME:GAS:foul vapor]
[STATE_ADJ:GAS:foul vapor]
[DISPLAY_COLOR:6:0:1]
[MATERIAL_VALUE:1]
[SPEC_HEAT:7]
[IGNITE_POINT:10508]
[MELTING_POINT:2]
[BOILING_POINT:NONE]
[HEATDAM_POINT:10250]
[COLDDAM_POINT:1]
[MAT_FIXED_TEMP:NONE]
[SOLID_DENSITY:500]
[LIQUID_DENSITY:NONE]
[MOLAR_MASS:NONE]
[IMPACT_YIELD:10000]
[IMPACT_FRACTURE:10000]
[IMPACT_ELASTICITY:100]
[COMPRESSIVE_YIELD:10000]
[COMPRESSIVE_FRACTURE:10000]
[COMPRESSIVE_ELASTICITY:100]
[TENSILE_YIELD:10000]
[TENSILE_FRACTURE:10000]
[TENSILE_ELASTICITY:100]
[TORSION_YIELD:10000]
[TORSION_FRACTURE:10000]
[TORSION_ELASTICITY:100]
[SHEAR_YIELD:6600] used high salinity ice
[SHEAR_FRACTURE:6600]
[SHEAR_ELASTICITY:100]
[BENDING_YIELD:10000]
[BENDING_FRACTURE:10000]
[BENDING_ELASTICITY:100]
[MAX_EDGE:500]
[ABSORPTION:100]
[IMPLIES_ANIMAL_KILL]
[ROTS]
[BLOOD_MAP_DESCRIPTOR]
EDIT: Also needed this body detail plan:
[BODY_DETAIL_PLAN:MELTING_EGG_SACK_PLAN]
[BP_LAYERS:BY_CATEGORY:EGG_RACK:MELTING_EGG_SACK:1]