You can do this. It's a better idea than a hybrid IMHO (as a hybrid owner!) because you can still use big beefy tires and go on trails. That doesn't work too well, if at all, on a hybrid.

If you have a fast-rolling 29er tire, it will feel fine on the road. I would use something with a near-continuous center contact patch. They roll best. Of course, if you're just out for fun and fitness, it really won't matter much to just ride whatever tires the 29er comes with.