So here are some thoughts, after reading just the op.
I fully love and endorse this idea! I remember sometimes when trying to organize pickups we'd only have 4/5/6 players so we'd often times come up with mini games, which was more often than not simply a half hazard form of practicing O v D.
Also, to me it was always fun to do small quick game types. 3 Minutes, 4 v 4 Nivo for instance can actually be quite strategic and intense. Until servers come though you'd have to mod something.
In Terms of Cap Off's , I usually like to do a minimum of 2 v 2 for O v D trade off's. Much like you said you get x amount of minutes or x amount of tries, then you switch roles/sides and keep score. You can even spice it up with rules like Outrider only (lt?) No Hof, No OD, You must make a Flag pass before scoring, LD can't grab to stop cap. Etc.
If you were to put people into brackets, I'd say do 2 v 2 minimum 3 v 3 max, with a quick time limit and O v D that switches a few times, and play best out 7.