from the point of view of RF (e.g. PSK)
Consider a QPSK scheme with one bit per half-cycle of a sinusoidal carrier. Bit periods begin at every zero crossing of the carrier, which is phase modulated such that 0 bits are modulated in phase or antiphase with the carrier, and 1 bits are modulated 90 degrees different. A transition from 0 to 1 or vice versa is modulated as a delay of the carrier by 90 degrees, such that the signal's frequency never exceeds that of the carrier.
The DQPSK that I just described is equivalent to MFM, assuming I didn't make a mistake. I can draw graphs if you want.
As far as round-tripping data
I wasn't considering that. Mostly I had one-way applications in mind: NES to PC, possibly to Internet.
a sufficiently constant feed rate of magnetic medium (i.e. minimal wow or flutter)
Not magnetic, but there's Such as recording your TV with a PC or phone mic, or hooking audio out into a PC's sound card.
If you're suggesting the use of more complex waveforms, such as multiple carriers, that might be harder to modulate in real time on a 1.79 MHz 6502 than, say, 4800 bps MFM.Note to self: lidnariq is pro-split