As mentioned, this is something you need to take up with the network. There is no technical reason these channels can't be registered.
However, FWIW, you can send utf-8 through a browser. In fact, pretty much every modern browser will default to sending data encoded as UTF-8, so you don't actually have to "know how" to input the data in any special format. And it will be the same format mIRC uses-- UTF-8. If the data is re-encoded on Undernet's end, that is their problem. If you noticed that they are re-encoding UTF-8 data in some crappy Latin-1 encoding, you should tell them, because it means people aren't getting the channels they are asking for, and not the other way around.
