Does anyone know how to do something similar with MUSHclient? I suspect it'll involve a plugin to modify the text coming in, but if anyone has an easier solution while I'm working on that, it would be lovely.

I figured it out. MUSHclient has a Hyperlink command that functions as a colournote. You just have to use three seperate commands, and gag the output line, blah blah blah. But it doesn't involve plugins.