I am happy to report that this problem appears solved on my system.
The issue turned out to be i/o errors with a 4TB hard drive attached to a Mac Mini via Firewire (running Snow Leopard and media files shared out to Win7); switching to USB solved the problem. Oddly, this problem never affected media playback, just the playlist issue and, as I just discovered, a delete denied permissions error on both JRiver and on OS X. Nothing to do with JRiver MC or JRemote.