[drunken monkey]

i've used it but not the extent that i would say that i'd be the best source of advice.

back in my study days, i used it on several occasions when i needed to make quick molds.
as i implied above, i used to first make a 'perfect' first positive cast (usually using alginate for the neg - plaster or the clear resin positive) and then use the hot vinyl to make negative molds from the cast positive which i KNOW won't melt or warp.

admittedly, this was mostly down to my always being a bit nervous about pouring hot liquid over the original, especially when the original was my face or my hands...

from what you say about packing out the void spaces with plasticine (i'd recommend using proper red clay), i can't imagine that the plastic would warp. I can't how-ever, claim to know at what temperature that the plastic melts.