Okay, I now know how to do this, although it will take me some time to implement.
This case is for having an acoustic drummer set the tempo on the RC 300.
Get an piezoelectric acoustic drum trigger and mount it on the snare rim.
Hook the drum trigger up to a midi percussion module with a midi out that has configurable midi messages.
Edit the outgoing midi message from the percussion module to be CC#64 with a value of 127.
On the RC-300, go into Memory Edit | Assign Controllers. Find an unassigned controller, like 8. Set it to ON, Target to MEM Tempo (tap), and Source to CC#64.
First caveat: Don't use this to trigger the Rhythm Track or it will restart on every beat.
Second caveat: The drummer will need a way to turn off the trigger when he's not hitting every (other) beat on the snare.