> I remember some sound demos on the PC that emulated polyphonic sound by fast/audible switching between notes in a chord

Didn't know that about the PCjr! I'll have to see if I can find some examples of the generator online. Polyphony by rapidly switching between notes in a chord was also one of the common tricks on the SID chip for the C64.

Even though it has separate channels, you could use get away with a single channel to play triads by arpeggiating extremely fast between the three notes.