@CammoXD : yep, brilliant.

The ONLY issue with this is when you want to update settings, but that is hardly a problem, just have to un-tick Read Only before you log out with the changes in place and reapply it after client is offline.
Just to make it clear: it's not just about consciously changed settings but also about a few things that addons save automatically for you.
So for example UltimateMailMod saves the name of your new toon when it first uses mail. For its name to be saved in "your characters" you would need to make the file writable for a moment before logging out of the new toon account.
Also make it writable e.g. before logging out from your AH alt to make sure your last prices and history of prices is saved. (Which actually might be a nice practice anyway for people _without_ multiple installations, but running a few copies of the same client in parallel - it's annoying if e.g. your last AH price is not saved because you logged out of the AH alt while another client was still running...)