I want to make and cast a bunch of "skeleton" parts (chests, pelvises, bits of legs, arms, skulls and so on) so I can wire them together at the joints to make armatures for sculpting on (rather than building a contraption from wire every time). If I cast these in resin, does anyone know if I could build directly onto that with Milliput? I would rather not go to the expense of buying all the moulding and casting stuff only to find it won't work.

Yes, you can. I use wire armatures and resin dollies all the time and only ever sculpt in Milliput.

Resin + Milliput
It works.

Great! Thanks!

And that's straight from the horse's mouth [sort of].