This may be too simple, but how about looking on ebay for a ISDN codec that CAN do stereo like Telos Zephr or one of those dreaded Musicam prima 120s, etc? I think you'd be much happier if you're still forced to stay with ISDN service out there. Have you checked around to see if there's any good wirelsss internet providers in the area?
Yes, you CAN do it with ISDN, but it wouldn't be pretty. I would think you could somehow find a ISDN modem made by Adtran or someone like that which would provide a nice ethernet connection. If you had that on both ends, dialed up from one unit to the other unit, and had null modem cables, you could probably get it work as a point-to-point. Buying something like a used ISDN codec would be so much easier.
Good luck!