There's actually some 6mm Brets floating around somewhere, the game was originally going to be done in plastic. I guess that the shelved that idea when they compared the costs of doing all the races in plastic compared to metal, and the plastics ended up being tBotFA.
And of course don't forget the ultra-rare Project Norse stuff. Whilst none of it was sculpted 'in-house' by GW, the models were cast on the then studio casting machine and the mould then destroyed. You can't really get any rarer than that now can you? I think that I've still got a couple floating about, so if I do run a Warmaster event I'll probably try to cobble together a set as a first prize. Not that the sculpts stand up that well compared to GW released stuff of course, although Steve's 2nd sculpt was pretty good, mine did basically look like a gnome with a big axe...