Overriding the chat window in an extension

[Valarian]

I've been trying to override the chat window control using an extension (to reduce the screen resolution dependency). However, no matter what I try it's complaining about the window references already registered in the ruleset rather than using the overriding values in the extension.

What are the rules on the getting the extension to override the ruleset?
Is there a way to unregister controls that have already been registered?