Thanks for the thoughts Markku.
By hacking the firmware, I'd decompile it completely down to the C it was written in and change it there, then recompile it, so that would avoid the addressing issue. Though the checksum check could be an issue, or maybe not as the installer uses the version number in the filename to check against, but I don't think I'd risk it, too much of a pain to remove the MPU to reprogram it :).
I'm thinking of setting up a serial man-in-the-middle Arduino to pass all the serial comms with the exception of the problem causing characters? Never done it or sure if it's possible, but it would probably be the easiest option as I've never written a driver before and reverse engineering a MPU's code is no easy task.

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