thanks for asking. I'm not able to reproduce it for the lack of a 
MythTV setup. However, if P on a regular keyboard works, so should 
the Remote Buddy action. Except, there are many different ways to 
send a keystroke on OS X.

So, I've created to variants for you to test:

http://www.wapapp.de/MythFrontend.rbscriptedbehaviour.zip 
(sending a single keypress between press and release)

http://www.wapapp.de/MythFrontend.rbscriptedbehaviour_2.zip 
(sending a single keypress between press and release, but does send 
it to MythTV as a global keypress rather than a local one)

Please let me know, if both, one or none of them solve the problem.

Best regards, 
Felix