Some things HUD-related can be changed, some things can't. Why some things can't be changed has little to do with it being a client mechanic and a lot to do with the "mess" that is the state of the server.
For example, the reason that we can't allow players to customize the color of their interface is because until maybe a year ago,
every single item had its own icon
with the button graphic in it. Like so:
In order to allow custom colors, we would have to go back and replace ... I think last time I checked, over 800+ files.
The biggest hurdle I can think with customized controls is how "hard-coded" the positions of some buttons are. For example, some other scripts(like the archery button) bases its position off of the sword button. You'll probably find quite a few of these situations on the server, and it could be very hard to track them all down.
That's not to say I haven't been interested in doing so, but it would probably be one of the more difficult things to do. Would have been way easier for these sort of things to be established in the beginning, rather than years down the line.