This topic was discussed at the Cornell conference this last May.
I was unaware that there had been so much effort put into this area
of product development.  Bob Berkman spoke about these developments
(I believe Rex Lawson chimed in).

Bob bravely demonstrated the idea, using a player to accompany a record
of a famous opera singer from the 'teens.  I thought it was a doomed
endeavor, given the timing issues, but Bob pulled it off wonderfully.
I just don't know how much practice it took to get to that point!

It obviously was not a product combination that succeeded, but that
wasn't for a good deal of effort on the part of the player industry.

Paul Johnson