I've thought about this before. Only two real issues, both minor:
[1] over-draining the batteries - not even sure if that's an issue with the RC packs
[2] low battery indicator won't trigger til the pack is way low (see [1])
I just don't find my 6V, 7AH to be all that big or heavy - not to mention it's paid for and built - so I never pursued this avenue. Great idea, though.
Nick - if you do this, let us know how it works out!