I think it’s just the same desktop file. We might easily allow flagging particular applications as autostart and copy or link the file in the right place.
We just need to figure what’s the proper way to do that for all users. Would it be okay to drop the file in /etc/xdg/autostart or does it have to be in the user’s home directory? If we need to have it on every user’s home, then one option would be having the user daemon (snap userd) doing that.