A few more experimental results:
Modding a creature to give booze when milked results in a bucket with booze in it. This will never be transferred to a barrel. Nor will dwarves drink from it, even when thirsty, but they will dump it on the floor when they need to empty the barrel to milk the creature for more booze. It seems that alcohol in a bucket just doesn't work. (It may be possible to cook the milked alcohol, however - I'll need to test this) ETA: Nope, can't cook it either. So far it seems milk is the only liquid you can usefully extract.
Modding a creature's muscle or skin to be shearable results in the creation of a mutilated corpse at the farmer's workshop (and no apparent harm to the creature!). This mutilated corpse is useless and unbutcherable and gets dumped on a refuse stockpile. I'll try and see what happens if you do this with shell instead. ETA: It works! I was able to 'shear' a shell-cow's shell and use it to make shell crafts.
LAYS_UNUSUAL_EGGS does not require a creature to be female, and in fact does not seem to prevent normal breeding - creatures with LAYS_UNUSUAL_EGGS and FEMALE will give birth normally in addition to laying the unusual egg. Likewise, a creature with LAYS_EGGS but not FEMALE will always lay infertile eggs.
If you give a creature multiple LAYS_UNUSUAL_EGGS lines, it will pick one at random each time it lays. This enables you to make a creature that lays a random gem every season.