Well, yes, a multiple layer board is nightmare even if you add an extra component somewhere because of its nature. I only suggested the internal pin header, as somehow the external cartridge connector must be connected to the PCB anyway, so it's maybe easier to provide an internal pin header very near to that point. All other places / solutions would require re-route the PCB design and maybe using more layers, as far as I can imagine the situation, though I have never seen the whole schematics of the M65 and/or board design at all ...
:: @LGB added on 05 May ’18 · 17:02
Also, adding an "internal cartridge pin header" would allow some personalization of adding always-used external hardware to the system, in the form of "external yeah, but well, internal to the case" :-P I would opt to try an SFX Sound Expander, home made, the OPL chip only, for example, as en extra.