I'd love mounted character minis to match the standing minis. For exactly the reason DreamingDeamon said. Mounted combat in 3e is hard to do with the standard figs you have. I'd tend to put the character mini on the line in between the two squares it's taking up, but mounted figs would be much better.
I wouldnt recommend the leg bits thing though. There are many players who dont like converting because it's a pain. Making some new models with both a mounted fig and a standing one in one package would work. Although that's probably not as good an idea for a character peice because once you start putting too many peices in a blister it becomes hard to make out what the miniture will look like when put together from the just looking at the package. Going with just doing mounted alternates of characters that they already make would probably be the best bet, and very very cool.
