hrmmmm
Well here's the thing. The rule about allowing outside collaborators is really something that came out of a discussion in the last WCRG. Some artists expressed that since they had access to session musicians, they should be able to make use of that. I had previously indicated that that kind of thing wouldn't be allowed, but they made a good case for it, pointing out that using session musicians was a lot like using specific sample libraries; i.e., just another tool in the toolbox.
When you're competing in these competitions, it needs to be YOU that's competing, not you and all your friends. In the case of Doubles, it's YOU and YOUR PARTNER. I realize that people like to work with others, and that's great, but there's a certain point where I just have to say that you need to be working on your music yourself, and not relying on collaborators all the time.
So I'm going to say "no." You can't bring in outside collaborators unless they are essentially just session musicians/vocalists. Don't bring in someone to do part-writing for you, don't bring in someone to write your drum tracks, don't bring in someone to write a killer synth solo, don't bring in someone to spruce up your production. YOU need to bring YOUR A-game, not everyone else's.
This goes for both Singles and Doubles.