It is sufficient to provide the synthesizer itself.

It is also the most complicated part, and most likely a violation of copyright and whatnot (and much more high-risk than OPL for which patents have expired and the chip itself is no longer produced). That will most definitely not happen. I mean, the only way I could even potentially consider this if some external project writes that synthesizer. I surely will not write that synthesizer by myself because that will effectively halt work pretty much any other more important OpenMPT feature for a considerable amount of time.

Implementing a transfer feature to copy patches from or to the Reface DX itself would be a useful feature.

There are thousands of synthesizers out there, I see no reason why OpenMPT should single out any specific synthesizer and add a programmer for it. It would make more sense to implement this as a VST. Sonic Quest's Midi Quest ediors would be a good choice for that but it's a bit on the pricy side. Ctrlr might be a viable free alternative.

Low-income users may have difficulty finding $300 in their budget.

There are many, many free alternatives available as plugins already. Try Dexed, which is a faithful recreation of the Yamaha DX7 and can be loaded as a VST in OpenMPT.