I would simply try this, but I've had my mx-500 crash a couple times and need a total reset, so I'll ask first...
I understand a device key can have a IR code taught to it by pressing "page" right after "learn", will this still allow me to keep my press-and-hold macros I already have on my device keys?
and... would the device key output always happen when the device key was pressed, or if I tried a press-and-hold, would the device key signal not go (this wouldn't make sense to me...). I use the press and hold macros on the device keys to allow the inputs of my tv and av receiver to remain in sync, and then land the mx-500 on the correct device, and I'd hate to lose this feature.
I hope I've explained myself well enough.

That's correct. You can have both a single command and a push-and-hold macro on the same device key. The single command will always fire, even in conjunction with the macro.

Mike
Surfremotecontrol.com