Great question. Is the solution really so complicated? Where do you disable guest users, for a start?
Why not disable other members who do not have a certain status? Just like you can do with regard to adding groups or posting video, creating polls, etc?
If such an arrangement or settings do not exist maybe someone can create a module which would give Jitsi a few more security controls.
If a community was large - dozens of conferences happening simultaneously would surely slow the site down. Some ability to schedule things would be helpful.
What a sick feeling if trespassers were using your site and your bandwidth to do or to plan to do things which you don't approve of. Or things which... might hurt others? Yes, these same "guests" could register and likely get away with it... Still a little more control or the ability to moderate might be useful.
Exactly, you can have a lot of control if you lock it behind a paywall (premium level), for example.
Controlling that on UNA's side that's not a big deal, just set the privileges according to the user level, but all that doesn't transfer to the actual Jitsi server - it will still create you a room if you visit the correct URL regardless of your membership status within the local UNA installation.