Unfortunately, we don't want to have custom maps on our current TF2 servers. While I love the idea, and I'm sure a lot of people would enjoy playing on a different map for a while, it's not what these servers are for. We don't want our players to have to download custom content to play on our servers; there are enough servers out there that force you to download 20 custom sound files and models. We just want to keep everything simple and as "vanilla" as possible.
The other reason is that we're using Valve's quickplay settings, and custom maps are not supported.
It's a good suggestion though. I guess we could maybe have another server with custom maps/content, but for now we haven't seen enough demand for such a server.