What does the Audya manual say is the trigger MIDI event for a fill?

Most arrangers, it is a PC#.

But you CAN trigger a fill with a footswitch, can't you? Why not stick THAT on top of the PK, or just to the RH side?

Way I see it, it is unlikely that the PK can send PC#'s on a different channel to the channel that the bass notes are sending on (could be wrong, though), which is what they will need to do so for sure to trigger fills. Secondly, there is WAY more than one fill in the Audya, so how does ONE button trigger the one you want?