@didier I don't see it actually manipulating the return address, so it might be portable (perhaps to 1.3+ at least). I've never seen this, but it seems when you return 0 from the bootblock, the bootstrap code will just continue booting at the code pointed by A0. That is then set up by the shown code, which read sectors 2..9 (offset $400-$1600) to be executed there. So they just needed a longer boot code it seems
Going to open it up again and see if anything got pushed out of its connection when I closed the case yesterday. Trying to get it back to a working condition again and go from there.
Mouse should be coming in the mail today or tomorrow. Ordered a SCART/HDMI converter as a backup to the piggyback card moving forward.