First of all, you'll need to divide the teams in your league into two roughly equal groups. If you have divisions or something similar use those; otherwise divide the group into goods and evils or something like that.
Each team representing their (division, ethos, or what have you) will be composed of several players from each team, i.e. an all-star game. We used twelve-man all-star rosters so that every player would be likely to play in the match. We also made sure that the teams were represented positionally:
4 linemen/beastmen/hobgoblins/longbeards/skeletons/zombies
2 throwers/passers/runners/Phoenix Warriors
2 catchers/Gutter Runners/Ghouls/Lion Warriors
2 blitzers/Bull Centaurs/Dragon Warriors/Wights/Wardancers/Storm Vermin
2 chaos warriors/cd blockers/witch elfs/troll slayers/troll/treemen/ogre/black orc/rat ogre/mummy/saurus
Stunties were eligible to appear as catchers or linemen and beastmen were also eligible as blitzers. We took the top players in SPP at each position (but no more than three players per team -- if more players were top at their positions, their coach involved got to decide which three went and then a replacement was chosen from another team). We had four coaches per ethos, so we said each coach sent exactly three and that worked rather well.
Selecting the team was only half the fun. To play the game, we had all eight coaches involved. Each head coach (the coach of the top team in his ethos) designated one of the coaches to move first, and that coach had to take whatever action he was going to take with all of his models before the next coach could act. This made it a little harder to coordinate than a regular match. We also said that the coaches could not discuss strategy during their turns and enforced a strict four minute limit per team turn. The head coach could declare any of the coaches on his side 'done' and let someone else act.
Each participant got all the SPP they earned plus a bonus MVP for appearing in the match. No winnings were awarded, but each coach could bring their team's apothecary for their own players only. Each coach also had one RR to use for his team's players, and that coach decided its use -- not the head coach.
The teams were spectacularly ineffective (the evil ethos team had particular trouble moving the ball IIRC) and the big-time orc dirty player was ganged up on and killed (after being, um, accidentally left in a bad position by the head coach calling done on his owner).
Stupid fun.
B