Changing the UI certainly isn't the most straightforward with the mod system. For my UI mod I went the direction of building the entire UI bar all by itself (and as a new/separate file). Then I modded script files to strip out the old components and patch mine into place. You could also do it by directly modifying the original UI, which would reduce a little of the patch work (although would still need to edit a number of things in the script to make it compatible). Going this route becomes a little trickier if multiple people/mods want to change the UI.