I think the best idea would be the PDS route.  I had this idea not too long ago, though I don't know how feasible it'd be since I'm not very knowledgeable with this kind of stuff.  Have a PDS card with 8 RAM slots on it.  Have a device driver that maps the memory for you.  Or do something like the 128k/512k/SE upgrades and make it a RAM disk instead, then just use software such as Virtual Compact (or a driver in ROM) and use the RAM disk as virtual memory storage.