Thanks. I have extensive experience with apoxie sculpt and apoxie clay (I buy both by the pound-jars), and I've used paperclay and sculpy air too. All clays have pros/cons, and what I need for this particular project, they won't do.
If you're familiar with sculpy's 'bendy sculpt' clay, that is what I would prefer to use for what I have in mind. It adheres well, sculpts well, and after it is cured becomes rubbery instead of rigid. It's what I use for things like tendrils and tails, because it won't fracture or break like apoxie, and it bends with whatever I'm sculpting it over vinyl (like a soft G3 body, or a MH head). It's also significantly lighter than apoxie, and will not make a doll with head mods more likely to topple off their stands and break.
Example:
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RequiemArt.com, on Flickr
The tail, horns, and tongue can all be bent (the tail can even be posed!), when something like that sculpted out of anything else that long and thin would snap with even careful handling.
I'm 90% sure that popping a MH head in the oven to bake will be just fine, BUT getting it on the body requires more squeezing and wiggling than I would like. So I'd have to bake it with the body. And the body is what I'm not sure of withstanding 275 degrees.