just tracked down a pretty good tutorial for casting in resin using a silicone mould, exactly how we're thinking of doing the replacement heads. the site is an Italian toy collector's site and the author, Alessina. I've used google-translate to stick it in English:
and heres a second tutorial by the same author on making two-piece molds for the same technique:
Two-Piece Mold Tutorial
there are a few bits where some of the translation is way off but it makes it more interesting to read! It's a really clear tutorial anyway. Although I'm sure using foam board for the mold case is just as easy as lego.