I can't say for sure why that one is called a corset top since there isn't a pic of the back, but I've seen others like it on Ebay and they have lacing up the back. I think grommet tape (I'm not sure that's the technical name) was used and then some sort of ribbon is threaded through the holes.
Maybe try resin, which is this epoxy-like stuff. I bought mine at A.C. Moore but haven't tried it out yet. It looks thicker and sturdier than most varnishes. I just bought a magnetic purse closure at Hancock Fabrics last night.
I've been eyeing some really cool painted purses on Ebay. The artist has used vintage vinyl and leather purses. I don't know if she sealed them; who knows, maybe the paint will flake off the first time it rains. They LOOK really cool though. If I win one, I'll try inspecting it for ideas.