There is a device upgrade in the JP1 Group and
with a JP1 cable and JP1 capable OFA remote you
could upload the device upgrade to an OFA remote,
test all possible commands and then learn them
to your MX-500.

The upgrade does not indicate any discrete commands.
It uses NEC1:0.

You might see if it turns On from Off with a Play
command. If so a macro for Play|Stop is Power
On and Play|Power Toggle is Off.