I've never really thought about it, but when I don't have the angle or the range to get a shot in the uphill leg, I try to fly lag pursuit and conserve energy so I will crest my loop a bit higher than the other guy. The problem with trying to pull lead going up is that you are essentially trying to pull a tighter circle (and working against gravity, as FLS said) and are blowing more E.
By being patient and conserving E on the way up, I think you can convert that into a lead angle on the downhill side. As you cross flight paths you should then get a shot opportunity, either as the other guy rolls over below you or by cutting inside his circle at the bottom (as Morf suggested).
I just made a (very) rudimentary image of this in paint, trying to use the same size loop circles, just to help myself visualize it. Of course the loops should be more egg-shaped (wider and flatter at the bottom), but the concept is essentially the same. Notice by flying LAG on the "A-side of the loop", you can come out on top and then naturally transition to a lead angle on the "B side" as you roll over, even while flying the same diameter loop.

At least I
think this jives with what Morf and FLS are saying. (Correct me if I'm wrong).
<S>
Ryno