Probably not my cup of tea as I`m not very competitive plus I don`t get out as often as I like. But I imagine it would appeal to quite a few players. I think Airsoft International magazine did something similar nationwide a few years ago, not sure how that turned out.
As far as I`m aware there are only about 6 sites in the south west, CAA, SWWG, KGB, Mad Mommas, MIA and the Bunker at Exeter. Whether or not all these would be interested I don`t know, I should think that each site could manage a dedicated team of 8 plus a couple of reserves.
There would have to be a set of rules which every site adheres to specifically for the league so that would take quite a bit of consultation between members, and also the games would have to be heavily marshalled by extremely competent persons I would imagine. Not saying that players are dishonest in general but when you introduce this level of competitiveness you may get the odd player not taking the odd hit and/or trying to bend the rules to give them an edge.
I`m presuming that the games would be a mix of home and away games with points awarded for a win/draw etc. Not sure how you`d do the selection of who plays who, maybe picking bits of paper from a bag, or a case of each team plays every other team to start with, then whittle it down ?