I'm pretty sure I can speak for the team on this one;
Though overall a nice idea, the complication of the codebase and CPU and i/o of the server being sent each users map information, even if a hash of the clients maps, prevent this from being something that would be added into ULib directly.
The additional i/o every time a player joined would probably bring the Gmod server to it's knees. (Stream snapshot errors, anyone?)
It's potentially a module release for someone that wants to dig into writing one.
However, by the time most servers would have all the clients uploaded map hashes (if that even worked, Gmod's data streaming capabilities are notoriously flaky, especially with other poorly implimented addons being able to break all others), comparing them against it's own, then showing the list to the admin requesting it, you could have invested in a decent fastdl server, switched maps and have had the client download the map already.