that would make sense but relying on B3 solely to track the 'under demo' state is not reliable. One with the rcon password could issue the "/rcon stopserverdemo all" and B3 would not be aware of that.
It is the same issue as for the mute command.
For current state of the game server I would not put too much effort in trying to go around game server 'lack of features'. I will wait for UrT 4.2 and its autoupdater. Then I have hopes that FS will be reactive in distributing frequent 4.2 patches through the autoupdater. One of these updates will have to be for demo state tracking
As for predicting the name of the demo : you don't need to predict it as the startserverdemo gives it to you.
I chose to put current server demo stuf in a plugin (as opposed to in the iourt41parser code) as these features aren't on most servers and are very young (bug prone also by definition).
With 4.2, I plan to add new B3 events specific to UrT and server side demos. Hopefully will have events for demo start and stop.