No current production Boeing Commercial Aircraft allows the selection of reverse until the air/ground logic has been satisfied. In the case of the 737 the use of radio altimeter may mean you're not quite on the ground - but deploying them before actual touchdown is not approved and can be very hard on the aircraft (and perhaps your flying career). When the reversers deploy, the efflux kills the lift on that wing (as we discovered very unpleasantly on Lauda) and the aircraft drops like a rock. Plus, if one reverser fails to deploy for some reason you're also going to get a nasty roll at a bad time.
I'm with sonicbum. While it was possible (and even approved) on some older aircraft types, it's a very, very bad idea to try it on any current Boeing.