The raceglosses can include any wood, any metal, any stone, or any plant.
When a creature with a racegloss spawns, the game chooses an appropriate material from the appropriate list. The creature is then labeled appropriately; for instance, in Relentless Assault you can encounter Tower Cap Treant Wrestlers and Ashen Treant Crossbowmen. I also at one point gave orcs RACEGLOSS:METAL, and would have things like adamantine, rose gold, and bismuth orcs show up as well as other, less exotic metals. It does appear to be completely random, though, so I guess you could do a sort of random loot system... Hmm...
Y'know... You could give a race one of the matglosses, for instance stone goblins, make a special "toy" called, say, "goblin loot sack," then give the creature ITEMCORPSE:TOY:LOOT_SACK_GOBLIN:STONE:USE_RACEGLOSS, then make a number of custom reactions that each turned a specific variety of loot sack (material and race) to generate a specific set of loots. You could even have every smelted "goblin loot sack" generate a goblin skull and 6 goblin skulls so it's truly like they're just carrying something extra.
...Although since it's been confirmed, both by testing and by Toady, that smelter products' first numerical value is a %chance of getting that item, so you could just do random minor loot that way, too. Major loot, like abnormally-powerful weapons that you normally can't produce, would probably be too powerful to have even a 1% chance of getting them be balanced.
Now I'm confusing myself. I need sleep