Paul,
It would depend on the gear reduction method, but I don't see why it wouldn't be possible to run one in reverse. 'Reverse' can be done by switching or by using an ESC, switching is simpler and a lot cheaper really (unless you just happen to have an extra ESC laying around). A 'limit' switch isn't all that hard to 'do' either.
There's almost always more than one way of doing things. Pick the one you are most familiar with or that you have the materials for.
- 'Doc