I think that questions on mathoverflow are in most cases on topic here. Does it make sense to have the question on both sites, with a complete answer on mathoverflow and here it appears as unanswered?
A similar question was raised here a few years ago. I think the subject has changed a little since then as now MO is part of the SE network.
What do you think? I think closing as off topic (although not totally true) is the easiest way to get the question from the unanswered queue. Of course another possibility would be to copy the MO answer here with a reference. This is the way some of us have done it before to mark the question as unanswered.