Some guy who was a respectable MSX hardware guru in the 90s once told me that the turbo R mainboard (maybe only the ST, cuz that one was first, he didn't mention though) is partly prepared to have an intergrated MFM interface...
I wonder, with the knowledge of today, if this still stands.
I read nothing about this in the wikis, so maybe it's missing, or maybe it's a fairytale.
/me also heard this story, but I truly don't think it's true. The MFM interface was already very outdated and thus quite obsolete around that time! For those who still are looking for evidence it might be true: try searching for traces of components of the ST-506/412 interface...
Though I might be completely thinking in the wrong direction here and the MFM implementation might be true... Not as a hdd interface, but as a floppydisk interface to support the then relatively new HD (High Density 1440kB) format. IBM implemented it only a few years (1987) earlier in their PS/2 machines.
