Going out on a limb here.
You could use the same control that the conference bridge from Kyle H. uses.
The source number callback entry P1234 for example.
Then you could make a setup for reception to dial into the VM and change it and all transfers look at that and only transfer if the number matches and if not they send calls to reception instead (or better disconnect immediately)
every time a room clears out they change the callback entry to anything but 1234 and transferring is not possible any more and ass soon as the room gets occupied again they change it back.
To make it easy create 2 shortcodes
1. they dial the shortcode enter the extension - in the voicemail it will take that information and sets the P1234
2. they dial the other shortcode enter the extension - the voicemail sets the information to P4321
that would however not check the outgoing calls

except if you send their outgoing calls through the voicemail and check if the entry is P1234 and then allow the calls to go through.
Lots of heavy lifting until it is set up but then you are good to go and have a happy customer.
Joe W.
FHandw, ACSS (SME)
"This is the end of the world, make sure to buy your T-shirt before it is too late"
Original expression of my daughter