Would something like this work? If so, check out hardware hut.com
Joe
"When the horse is dead, GET OFF."
That's definitely the only option right now. As Jamie coined it, the "hatchback" method. Unfortunately the wife isn't a big fan, and is willing to forego the storage behind the mirror for the aesthetics she wants. But I know better. She will continuously lament the lack of storage forever, and what point is it to do a full bathroom remodel yourself if you can't solve your aesthetic AND design/storage issues?
So, I think you're looking for a high-tech solution like this:
http://www.zaber.com/products/produc...Dovetail_Slide
or this:
http://www.zaber.com/products/produc...hp?group=T-LSR
Pricey though
A motorized rack and pinion would also work, although I haven't been able to find anything that would be readily useable in your application, so some fabrication/engineering might be needed.
rack-and-pinion-drive-6037-2464743.jpg
That's pretty sweet, but yeah, obviously out of the budget! I don't need it to be motorized. Basically something like a drawer slide with a pull-in catch that can resist gravity. In the up position, the drawer slide (in its closed position) keeps the mirror held up against gravity, and in the down position, the slide is in full extension (ideally it would have a "soft-open" at the end of its extension). Or alternatively, since I don't think a "soft-open" exists, it could be flipped. In the up position, the slide is fully extended (would need a stop at full extension), and in the down position, it would soft-close.
But I don't think there are drawer slides like this...
Think about a double-hung window. The sashes slide up and down, just like you're trying to do with your mirror. The traditional design has pullies near the top of the frame. A rope or chain fastens to the sash, goes up over the pulley, and down to a counterbalance weight dangling inside the wall. A little friction in the frame holds the sash in either the up or down position.