I was just thinking about something sort of along these lines. The basic idea is to make a team that can play very effective tower defense, then have 1 or 2 quick pusher/tower-smashers to force a different dynamic than the 5v5 dynamic that the other team is presumably set up for. Fortify, mobility, pushing, towersmashing, and champs that can play effectively around your towers (on def) are key. You can keep 3 or 4 to defend your tower as needed, and then have 1 or 2 rushing around the map slapping towers...or quickly losing/taking the opposition if they split off a force to get you. The key questions are, "Can you have 2 people push down towers unopposed faster than your opponents can push down your tower being defended 3v5?" and, "Can you force them to fight in smaller skirmishes that you are built for, instead of the 5v5 that they are built for?"
Champs that I think could be rolled into such a team are:
Singe, Evelynn, Udyr, Zilean, Rammus, Yi, Sivir, TF (Singed and Rammus have great mobility and play great around your turrets)...
Maybe an Alistair and/or Soraka on the defense team.
I'm just curious if this sort of approach could be viable in high-end play.