Reading Niemeyer’s comment which is the authoritative one, they haven’t done anything about this issue so far because they’re focusing on providing a great end-user experience. If you want to distribute a snap you should distribute it via the official snappy store.
You’re more than welcome to host proprietary snaps there, @niemeyer could you explain how that works?
Oliver (Ogra)'s discussion in that thread is interesting and he’s produced a small, very hacky, script that would allow you to have a basic CLI for downloading and installing from a store using --dangerous. But this wouldn’t support snap refresh etc so it’s basically unusable for the end-user who would be using the official snapd workings anyway. The theoretical possibility of having an external repository is not the point that the snappy developers are focusing on, they’d rather people just use the official snappy store (which is much easier for the end user to use)!