Rick,
I realize it is not and should not be just tacked on for a sitewide PM. It would be a new feature and only a single bit would need to be changed. 0 not read 1 read (yes a little more than that but not much in regards to actual data size and hit to the system)
Most mail systems only store a single image even though a message may be sent to everyone in the company. When the user deletes the message it does not delete it, it just marks it as deleted by that user.
I don't see why (in some future version) that a similar idea could not be used with UBBT
