Since Yamaha style FM (actually phase modulation, but it doesn't really matter) requires both exact ratios and phase relationships to get that Yamaha "sweet spot" FM, it may be best to simply make a complete multi oscillator FM module for computational efficiency, ease of use and predictability of results.
Something like a 4 operator module, ala TX-81Z, with 4 AM and 4 FM inputs, shouldn't use much DSP power since it only consists of 4 plain sine oscillators. There is no need for any advanced VA oscillators (quite the opposite, in fact...). Then have a switch to select the 'classic' algorithms and maybe some unusual ones:
Edit to add: Even though Yamaha style digital FM is cool as such, it 's really not at all a VA/modular thing, and might understandably be seen as outside the Origin's domain... Personally I'd want the base functionality to be bug free before anything new is added.