Bumpbadump.
I'll be looking into how "Team Randbats" can be made soon, which is similar to your idea. Contrary to how regular Randbats generates teams from a single pool of Pokemon, Team Randbats would have different pools of Pokemon to choose from for each team. e.g. A player could get a rain team one battle, or a sun team the next. Of course, this isn't exactly your idea in the sense that players can't make custom pools/boxes of Pokemon for this without having to jump through hoops. Instead, I would have to implement them directly into our server's code and update it accordingly when people want them changed. Unfortunately I don't think it's possible to allow players to do it themselves without having to go through that. It would require quite a bit of coding that I'm not able to do. :(
