I like this idea and think you should go for it. I like the concept of everyone using the same style guns, and of all the types, blowback are the most common and available. If you want to play with cockers then start a mech league. I don't see the big deal, if it isn't allowed then you can't use it, case closed. If you don't want to buy a blowback then don't. Would you complain that you can't use a 250cc dirtbike in a 125 race? No you wouldn't because it isn't allowed.
The point of regulated competition is not to just grab the best of the best, it's to do what you can within given set of restrictions. If you don't like it then don't do it, just like how you don't race NASCAR or F1 because you don't have the right car, or how yo don't play PSP because you don't like the format.
So give it a try and see how it goes. Make changes and come back with a more refined version. If it doesn't work then big deal we won't write it on your gravestone.