Dave call Yamaha and maybe they will give you the system exclusive HEX script to turn off the hold pedal on the split left.

Send the system script via micro edit from the PA4x sequencer... or your DAW. It should change and be locked, so make sure you also get the script to turn it back on.

This type of system exclusive is available on Roland gear.. I am sure they can give you the data.

If you examine the midi reference info.. it may be found.