Microtuning Yamaha FM synths

>I've been racking my brains and searching the net for a solution to my
>quest for an accurate real-time microtuning synth.

Couldja clarify what you specifically you mean by "accurate"?

As for real-time, I'm inclined to think that any of the (few) machines
that support the MIDI tuning standard, like Ensoniq's ASR-X or MR series,
would be a good possibility.