I do not understand the logic.
You have one physical phone that is used by several different people based upon (your description) a time of day. What is not clear is how the different people relate to the mail box or boxes and their use. One solution might be to get a multi-account capable phone and then set-up a unique extension for each user and then use the global and specific office hours settings, forwarding rules, and auto settings such that:
Calls during the hours 8AM to 5PM go to extension 100 and then from 5PM to 12AM to extension 200 and then from 12AM to 8AM to extension 300 and back to 100. This may have an impact on how directories or digital receptionists and groups are set, so be wary. It is not explained if the extension in your scenario is generic (like tech support or order entry and it matters not who is staffing) or if it needs the individuality. You can then have the mailbox set for each accordingly (with or without PIN or even a common PIN) or you could set up another extension on the same phone as account 400 which vmail for any of the others would be set to "send voice mail to extension 400" such that it becomes a general or common mailbox that all (100, 200, 300) could access, but as has been pointed out, once read it matters not who read. This would accomplish - MWI indicate for all users.
Having an unregistered phone show as registered is not really possible.