No first hand knowledge, but an old coworker once told me using the Radio Altimeter (aka LRRA) dates back to the original 737-100/200. Apparently the 737 can go into a ground effect 'float' such that even though the main gear are on the runway, there is insufficient weight on the gear to trigger the 'Weight on Wheels' sensor, or to reliably spin up the tires. Hence the change to allow it based solely on the LRRA. But even with an erroneous radio altimeter indicating less than 10 ft., it would still be necessary to lift the piggyback lever before you could get reverser deployment. I sincerely hope there are no pilots out there that are dumb enough to try it...
BTW, IIRC, the same is true of the 777 - the T/R inhibit is based on LRRA, not Weight on Wheels.