1) Personally, I have no idea, but I think it's been done before.
2) Ditto - 5x5 gives you smooth shadows and almost perfect anti-aliasing. Though the new "soften" option in v12 might get you the same results at less than 5x5.
Of course, the regular A-Buffer renderer is fine for most things. In my experience, multipass gives you much nicer ray-traced shadows and motion blur, but takes much longer. If you haven't already, you might want to do some side-by-side comparisons and see if multipass is worth the extra time.
3) If you have enough space and CPU power, render and edit at the highest resolution you think will be used. You can always shrink it later. At work we experimented with "up-rezing" old footage to HD... Looked like crap next to actual HD footage.
Presets for 1080i and 720p are already in the render panel.
Also, to save time animate and render at 24 fps then use FinalCut or AfterEffects to do a 3:2 pulldown. It looks just as good and can save you tons of time and disk space on large projects.