Turns out, it's an air-curing, freely-shapeable silicon rubber that bonds to pretty much any surface easily. On first opening the foil pouch, it's got a texture like air-drying modelling clay or slightly damp dough. It's really easy to shape and finish.
I had a fiddle with a 5g pack and Number 12 (who started off as somethingsomethingcandy... must remind myself!) and this is what I came up with.
The website FAQ says that paint is at risk of flaking because of the flexibility of the cured substance... but I'm hopeful that won't be an issue on parts this small. I'm going to leave it overnight and have a good play tomorrow, and will just have to see what happens next! I've already got the materials for the rest of this girl and I'm confident she could be my most adventurous custom yet.