There is a lower-tier option, which was discussed in another thread a short time ago, which is to make a Trainer House feature which lets you put other people's teams in a folder and battle them (with the AI controlling them). It's pseudo-multiplayer, since you battle against other players, even if it's just their teams.
Trainer Houses are easy to make though. The entire $Trainer variable can easily be exported through .rxdata files and the pbLoadTrainer function can be adjusted to interpret that data accordingly. I've actually already made that :I
In regards to the other real-time online stuff: it has captured my interest. I've already started looking into it, and I'll probably continue to do so. It would be nice to have somebody here to bounce ideas off. I'm not willing to make an online functionality add-on all by myself, and give it away, though. I'm just keeping an open mind for now.