If the firmware update is distributed as a MIDI file, I suspect it probably contains the firmware bytes stuffed into System Exclusive messages inside the MIDI file.

MIDI messages use the high bit of each byte to indicate the start of a message (the Status byte), then the data bytes of a message must have the high bit of each byte clear. In other words, the data bytes can only use 7 bits, so each byte of the firmware has to be split somehow and stuffed into 7-bit data bytes. Perhaps this bit re-packing might prevent binwalk from finding interesting things?

The first step I would take is to use some MIDI file software to examine the MIDI file and look at all the System Exclusive messages.

I would compare the System Exclusive messages to the Kawai ES110's documented System Exclusive format to see if there were any similarities. (See the ES110 MIDI Exclusive Information PDF from Kawai ES110 - Support Download.) This will likely help identify some of the header bytes of each System Exclusive message.

I would look at the data bytes to see if I could tell what kind of packing method was used, and if I could tell if any other kinds of data sizes, start addresses, or checksums are present.

If we can extract and re-arrange the data bytes into the original firmware bytes, then perhaps running binwalk on that might provide more interesting info?