I had a bit of a dive into the MT32-Pi and MUNT codebase
MUNT has a set of hard coded SHA1 values for known good ROM images, so changing anything in the file will basically invalidate it. MT32-Pi is coded to retrieve the info directly from the ROM via MUNT with no options otherwise. So out of the box, there is no way to get it to show anything other than the ROM version string of a known good ROM.
You would need to compile your own MT32-Pi version which updates the CMT32Synth::ReportStatus() function in src/synth/mt32synth.cpp(176) to convert the selected ROM type to a readable string and send that to the display instead of the ROM version.
As a hack you could search for the hard coded SHA1 value of the ROMs you are using inside the compiled binary and replace it with the SHA1 of your modified version, I don't know if that will have any downstream effects.
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