István! You already looked inside the Scoretrack. Can you made a memory map of the 'a' Studio cartridge?

Cartridges also have the disadvantage that they do not support I/O ports. Memory mapped I/O is more problematic to emulate (unless it is somehow hacked into the existing SD card interface), and it would be slower to read in a modified version of midiplay, because paging needs to be saved, changed and restored in a high frequency IRQ handler. Assuming it is based on the 6850, input could be implemented better if the interface supported generating interrupts, otherwise the 6850 needs to be polled at a fixed high frequency, which does work, but uses more CPU time. With a VHDL implementation of the 6850, a minor optimization would be possible by making the data register return FFh when no new data byte is available.

I think like you, better an external expansion card. We have all necessary signals there. And bus expanders are cheap to build.

On the MSX approach they seem to connect the MC6850 /IRQ pin 7 to Z80 /INT, but then, who knows the origin of the interrupt?. I think that what is needed is to implement another read-only port for the interrupt status.

Does anyone have a photo of the bottom side of the interface PCB ?

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Do you have a photo of the top side with removed ROM ?

Not yet
This is my interface. It is have a additional hack, for simulating ROM from SRAM. (But not work properly, ROM contents always damaged...) My plan is restoring it to original version. I hope after Christmas will have some free time
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I will not read more from photos
I can guess, but it's easy to make a mistake.

It's DipTrace, up to 300 pins is free. https://diptrace.com/
DB9 in original interface is probably MIDI thru.

SRAM hack removed:

Being a new project, changing the pagination(and the MIDI.Rom code) to segment 06, the two devices(Midi+SD reader) can live together on the same cartridge.

Better idea put to Bus side. (06h used by EXDOS in SD cartridge)