The code for the PU22/PU23/PSone that's used in MM3 is identical to the Mayumi 4.0 code (in fact, it was probably copied from Mayumi) - the only practical difference is that the MM3 uses the internal oscillator and the Mayumi code uses the mechacon clock.
In theory, the internal oscillator should be fine - it's not terribly accurate, but the datastream is basically asynchronous serial, so it only needs to be close enough that it stays in sync for a single character. It got a bad reputation because some of the older programming software corrupted the oscillator calibration value that's stored in the last word of ROM and this made the chip run at the wrong speed.