Thanks everybody, for the input. Irulan, I think I pretty much do what you say you do: have the seat in a comfortable climbing position for the way up, then find a sort-of compromise for the roly-poly down. (It's rarely ALL downhill here; we don't have any lift-serviced DH kind of stuff.)

It seems to work pretty well, until I get to those certain pitches that sketch me out. Physically, yes, Alleyoop, I can get my butt behind the seat and my chest on it. It just feels awfully awkward, like I'm going to crack a rib if I still take a digger. I guess it also seems like steering would be compromised, and I kind of worry about my feet getting stuck in the clips. I don't have great flex in one ankle (old tennis injury) so maybe that's part of my head issue.

I don't THINK my bike's too big (I'm 5'8" and it's a Giant medium). So maybe I just need to practice that position--butt just off the rear wheel. Any suggestions on riding technique? I'm dying to conquer a rock face that I've been walking for far too long!