AHA! Figured it out!
The downside is that this method forces the offspring to look like the mother. You could probably randomize it somehow (I know that the old creature-creation script did this, so there's a way), but this at least serves to provide a way to check and see if your animals can carry a pregnancy to term:
Select a female unit in the unit list and enter the following commands in the DFHack command line:
[DFHack]# lua
[lua]# unit=dfhack.gui.getSelectedUnit()
[lua]# preg = df.unit_genes:new()
[lua]# preg.colors:assign(unit.appearance.genes.colors)
[lua]# preg.appearance:assign(unit.appearance.genes.appearance)
[lua]# unit.relations.pregnancy_genes = preg
[lua]# unit.relations.pregnancy_caste = 1
[lua]# unit.relations.pregnancy_timer = 10
Once you unpause, the creature will give birth to clone(s) after 10 ticks.
Of note: you have to create a new "preg" table every time you do this, otherwise the game will crash. I'll build a new script to do this more reliably when I get a little more time to play this evening (baby just woke up from his nap, so I'm back on the clock).
It might also be possible to make male animals pregnant with these commands. Haven't played with them overmuch.
EDIT: It does work for dwarves. They'll look the same, but their personalities, goals, etc, will be different. It appears that everything other than appearance and color are randomly generated.
ANOTHER EDIT: Last one, my little dwarf is trying to help me type. Apparently genes must not go through in a 1:1 ratio because I doubled checked and a baby made this way does not actually look like a copy of the parent. Rather, a few of the traits are kept and some new ones are rolled up. Pretty cool, actually. Haven't tried to make a male pregnant. I'll do that later.