I have absolutely no idea what engineering problems / solutions the idea would entail. I guess the interface between the mirrorless unit and the adapter (with mirror assembly etc) would be one difficult problem, plus maintaining tolerances of relevant registration distances.
I'm not sure why the marketing and support would be difficult? I would have thought it would open up loads of marketing opportunities. Two possible lens mounts to satisfy the old crowd (us) and new users, people who prefer mirrorless and those who want an OVF....
If it could be done well, and with a high degree of reliability, I think it'd be a marketing dream....a 'best of both worlds' solution.
But, I'm just speculating. That was the point of this thread, to throw around possible ideas for that 'different' FF