For posterity, I've figured out how to do what I was attempting--that is, different castes with different materials for bones. Instead of using materials, different tissues should be used instead. Like so:
[CREATURE:TEST_MAN]
[NAME:a test man:test men:test man]
[CREATURE_TILE:'t'][COLOR:0:0:1]
[CAN_LEARN][CAN_SPEAK]
[PREFSTRING:versatility]
[EQUIPS]
[CANOPENDOORS]
[HOMEOTHERM:10067]
[SWIMS_INNATE][SWIM_SPEED:2500]
[CASTE:ONE]
[DESCRIPTION:A type 1 creature.]
[CASTE_NAME:type 1 test man:type 1 test men:type 1 test man]
[SELECT_CASTE:ONE]
[BODY:HUMANOID_SIMPLE:2EYES:NOSE:2EARS:HEART:2LUNGS:RIBCAGE:BRAIN:SKULL:MOUTH:GENERIC_TEETH]
[BODY_DETAIL_PLAN:STANDARD_MATERIALS]
[TISSUE:MILKBONE]
[TISSUE_NAME:milk bone:NP]
[TISSUE_MATERIAL:MILK_TEMPLATE]
[STRUCTURAL]
[CONNECTIVE_TISSUE_ANCHOR]
[TISSUE_MATERIAL:LOCAL_CREATURE_MAT:MILKBONE]
[RELATIVE_THICKNESS:2]
[HEALING_RATE:1000]
[VASCULAR:3]
[PAIN_RECEPTORS:50]
[CONNECTS]
[TISSUE_SHAPE:LAYER]
[SETTABLE]
[SPLINTABLE]
[BODY_DETAIL_PLAN:VERTEBRATE_TISSUE_LAYERS:SKIN:FAT:MUSCLE:MILKBONE:CARTILAGE]
[BODY_DETAIL_PLAN:HEAD_HAIR_TISSUE_LAYERS]
[SELECT_TISSUE_LAYER:HEART:BY_CATEGORY:HEART]
[TL_MAJOR_ARTERIES]
[BODY_DETAIL_PLAN:HUMANOID_RELSIZES]
[RELSIZE:BY_CATEGORY:TOOTH:400]
[CASTE:TWO]
[DESCRIPTION:A type 2 creature.]
[CASTE_NAME:type 2 test man:type 2 test men:type 2 test man]
[SELECT_CASTE:TWO]
[BODY:HUMANOID_SIMPLE:2EYES:NOSE:2EARS:HEART:2LUNGS:RIBCAGE:BRAIN:SKULL:MOUTH:GENERIC_TEETH]
[BODY_DETAIL_PLAN:STANDARD_MATERIALS]
[BODY_DETAIL_PLAN:STANDARD_TISSUES]
[BODY_DETAIL_PLAN:VERTEBRATE_TISSUE_LAYERS:SKIN:FAT:MUSCLE:BONE:CARTILAGE]
[BODY_DETAIL_PLAN:HEAD_HAIR_TISSUE_LAYERS]
[SELECT_TISSUE_LAYER:HEART:BY_CATEGORY:HEART]
[TL_MAJOR_ARTERIES]
[BODY_DETAIL_PLAN:HUMANOID_RELSIZES]
[RELSIZE:BY_CATEGORY:TOOTH:400]
[SELECT_CASTE:ALL]
[HAS_NERVES]
[USE_MATERIAL_TEMPLATE:BLOOD:SLIME_TEMPLATE]
[BLOOD:LOCAL_CREATURE_MAT:BLOOD:LIQUID]
[CREATURE_CLASS:GENERAL_POISON]
[GETS_WOUND_INFECTIONS]
[GETS_INFECTIONS_FROM_ROT]
[USE_MATERIAL_TEMPLATE:PUS:PUS_TEMPLATE]
[PUS:LOCAL_CREATURE_MAT:PUS:LIQUID]
[BODY_SIZE:0:0:3000]
[BODY_SIZE:1:168:25000]
[BODY_SIZE:12:0:70000]
[MAXAGE:50:70]
[USE_MATERIAL_TEMPLATE:SINEW:SINEW_TEMPLATE]
[TENDONS:LOCAL_CREATURE_MAT:SINEW:200]
[LIGAMENTS:LOCAL_CREATURE_MAT:SINEW:200]
[ATTACK:BITE:CHILD_BODYPART_GROUP:BY_CATEGORY:HEAD:BY_CATEGORY:TOOTH]
[ATTACK_SKILL:BITE]
[ATTACK_VERB:bite:bites]
[ATTACK_CONTACT_PERC:100]
[ATTACK_PENETRATION_PERC:100]
[ATTACK_FLAG_EDGE]
[ATTACK_PRIORITY:MAIN]
[ATTACK_FLAG_CANLATCH]
Unfortunately, they don't drop milk on death now, but type 1's have liquid bones while type 2's have regular bones. Liquid bones are immune to breaking, meaning it is darn hard to damage them with blunt weapons. I dropped one off the 4 level high part of the bridge in the arena several times, and he survived each time with heavy bruising while a normally boned test man will die after ~2 falls. Only a direct strike to the head, damaging the brain, will kill a liquid boned test man with blunt damage now.
EDIT: Enough blunt trauma to the lungs will also cause suffocation. Possibly heart trauma could kill too?