If such a rule were to be installed, I think it'd be most useful to say that someone has to has registered before the first piece of art was posted in the first art thread of that CaP.
I think recruiting is quite a lot harder if you have to tell a large group of people (especially ones you don't know) "Register there, and then in a couple of days come back and vote for an artwork, except I don't know yet which artwork".
That's a good idea, it makes it much harder to just tell people to vote for X since most people won't go to the effort if it will take a few days. Personally, I have to say that since I just came in at the beginning of this CAP, I would have probably left if I wasn't allowed to vote on anything. Yet this would just make them have to wait for the next poll. I hope it can be implemented.
Also, in response to Foible's, chances are if they are being recruited through Smogon they'll have a much easier time curbing it seeing as it would probably be through PMs and forum posts.