Err, no, this is not true. It's more likely that these clients will simply use a single encoding for all of their IO. As I pointed out, all of the clients I've seen on OSX only understand one encoding: Unicode. I highly doubt they behave the way you're describing (or wishing). Furthermore, there's no such rule that "commands must be sent using codepages". It's not in the RFC, it doesn't exist in reality either.
Every OSX client support both Unicode as well as other encoding standards. The users decides what encoding to use. There's a screenshot under the UTF-8 topic started by Jay_tea.
Most people don't care. But mIRC should allow the user to select the proper encoding if so desired. It does not. And people are not going to migrate to version 7 just for Unicode support.
