This is exactly what caused the master server spam a couple weeks ago
an exploit went unnoticed for probably over a year and then was heavily abused
This is something that cannot be fixed easily. Even by restricting the users to such extent will only trouble them even more. It's like putting a DRM on a game because it makes it harder to copy it. Users still find ways around to give out their add-ons. Users will still download any add-on, as long as it sounds appealing, without thinking if it contains an exploit or not.
However, thank you for bringing it up as that got me thinking even more on this issue. I've had something in mind to prevent people to abuse the system, and I'm still trying to figure out how to allow anyone to still upload freely, but limit the access to that uploaded add-on. Both relying on a fully automated system or a group of reviewers is flawed and is not a solution, but could be a tool to help out. The more power to the community, the lesser the chance of these exploits will be raised.
That's why I wont bother that much about restricting distributions of add-ons. It's a bit of a naive thinking, but I can't bother thinking about that in first hand as that isn't my goal with this project right now.
I can also mention that my code is fully open source, so anyone can modify to add such a feature if they think it's of utter importance.
On-topicI don't mind a collaboration. Either through automatic sharing of add-ons, or merging of functionality. Rather, if possible, I'd suggest that each one of us to work on a specific part of each system. Hosting(E.g. web, backend, chat server); Mod manager; Community functionalities(E.g. chat client, prefs). This way a huge projects will be split up in separate pieces, making it smaller and more specific people can work with each piece without having to bother about the other one(For the most part).