I've spoken with Kalmbach about this when I've met with them about my articles and video PDF project that I'm doing for them and they tell me it isn't likely to happen.
The market for MR on PDF (several thousand copies) is just not very large compared to the cost and labor issues. Plus just a scan of the pages would not be enough, you would need someone to go through all the pages and mark words to index ... the manual labor would be extensive.
Then there's the issue of payment ... years ago it was not unusual for magazines to buy an original article with "first time North American publication" rights, so now producing the material in PDF form could be considered a second time global publication, which is in violation of the original contract. Just tracking down all the contractual agreements and sorting out the legal issues, determining if the author should be paid more for the second publication, yada, yada ... it could be huge.
Unfortunately, much of that material was originally produced with no idea that digitial redistribution might be a future option so the original contract when the work was purchased doesn't allow for that.
Long story short -- no market and the production is fraught with technical and legal issues.
You can get back issues for a song on eBay and Kalmbach provides an excellent online index ... which is almost as good.
Joe Fugate
Modeling the 1980s SP Siskiyou Line in southern Oregon