I had a go at the Hulya mannequin a while ago and my husband (as always, he's so my ideas man) suggested that I put a zipper into one of the seams when I was complaining about how to make it work on of my stands. Genius idea for which I was very grateful. I stuck an invisible zipper in one of the side seams and can now use it either standing on a table or on a stand stolen from a different dressform.
I've been wanting to try the plaster/foam method for a while now, so thanks for the super useful info! However, I've discovered that there's something even better than the spray-in expanding foam from the hardware store. That stuff dries pretty hard and stiff, which means it's not corsetable. Smooth-On makes a two-part expanding foam that cures squishy, kinda like the stuff Uniquely Yous are made of. I've had an opportunity to try a sample and it's pretty awesome!