Well, so having now seen the service notes for the RA-50, I can at least say that there is no mention of a MIDI passthrough behavior, or similar, that would preclude the need for either SoftMPU or the hardware modification. This is hardly surprising.
Also, the resistor that I thought was in-series is just a pull-up, so it really isn't necessary to grab the MIDI signal where I'd initially suggested. Instead, pin 6 of either opto-isolator (IC44, 42) can be used. From a convention/consistency standpoint, it probably makes more sense to grab it off of the MIDI (SEQ) input, rather than the MIDI (KBD) input, besides.
Finally, and anecdotally speaking, I'm impressed with the output quality of the RA-50. I haven't done any actual, objective comparison tests between it and a second-gen MT-32 or CM-32L/64, but it certainly strikes me as being rather nice (punchier?). I can only speculate as to what the difference(s) might be, but I do note that the RA-50 uses Rohm op-amps, rather than the commonly-used New Japan Radio or Mitsubishi variety, and has a suffixed reverb chip besides (15239863-01, vs. 15239863).