The filtering of the All Notes Off message in the RA-50 occurs in the arranger section, which all MIDI input passes through. The sound section uses an actual CM-32L control ROM (despite having just half of the PCM ROM). The socketed, v1.00 EPROM is visible in Salient's photo, where one of my RA-50s uses a soldered-in, v1.02 mask ROM, by comparison.
I haven't seen the RA-50 service notes, but perhaps there is some hidden means of defeating the message filtering. Outside of that, cutting a single trace and running a bodge wire from pin 1 of IC43 to pin 1 of IC28 might be an acceptable hardware workaround. I have a sacrificial RA-50 that I can test this idea with.