I had lost cities installed on client and server, but client had the dimension disabled while the server did not. Attempting to connect to the server resulted in log spam on the client, and while the first time I was greeted with a Fatal Error on join, the second time I tried it basically hung and generated 96MB worth of NPE errors in my log until I disconnected.

The issue wasn't immediately obvious- is there any better way of handling not knowing about a dimension?

That's hard to do I'm afraid. Dimensions (and anything else for that matter) have to be registered on both sides (or not at all) to work. I can't think of a way around that really

The issue is less about it working and more to do with communicating to the user why it isn't working. I originally mistook it for another mod not functioning at all.

Even that is hard to do. There are probably ways but I currently don't know how. The problem is that Forge catches this error (and crashes) before I get the chance to find out