That's a neat idea. It makes me wish I had an acreage instead of a thin
city lot.

It doesn't take much wire, just enough to get a difference in reception angles
(sorry to those who know better, if I'm goofing up terms here!) and enough so
that each radio has slightly different reception. Then when you mix the
outputs, you may see less fading. I've never attempted a diversity set-up, so
I'm no expert, but it seems fun.

I'd like to put a few
hundred feet of #12 inside the garage ceiling before drywalling. It's a
peaked ceiling, resembling an A frame. I just don't know how effective
this would be.

So long as it's not going to pick a lot of man-made noise from your house
(appliances, dimmers, etc.), then at the very least it's a cheap and very
hidden antenna, and can hardly be a risk to try!

If that IS the case, I'd get an antenna that is the
electrical equivalent length to only the RISE above ground. It'd be a
shame to use hundreds of feet of wire to get ten feet of work out

Somehow I don't think that would be the problem. It sounds like more of a
pyramid-shaped depth wound loop antenna, just on a larger scale. At the very
least, you've got way more capture area than if you just used a ten foot long
vertical piece of wire, so it won't be a bust in that sense.
Try this site, if you haven't been there already, for hidden antenna ideas,
some antenna formulae, etc. :

http://www.hard-core-dx.com/nordicdx/antenna/index.html

There's a LOT of good info to pore over on the ol' HCDX site.
Best of luck (and if and when you get that pyramid antenna going, PLEASE let us
all know how it comes along!)-
Linus