[Phanx]

An addon would be useless for this purpose, because (1) the addon's data would be stored with all other addons' data in the WTF folder, which is usually deleted as part of installing a compilation, and (2) the addon would not have any way to know when you were about to install/update anything.

You could easily do this with a batch script (.bat file) on Windows, but it would require some customization by the user to handle (1) their WoW install path and (2) their WoW account name. Players with multiple WoW accounts would need further customizations to copy files from multiple locations. Overall I don't think this would be significantly less hassle (especially for users unfamiliar with batch scripts) than just copying and pasting the files manually.

An executable program could do it too, but would be much more complicated to write, and would have the significant downside of being an exectuable program, which (1) you would have to convince WoWI to let you include in your compliation package, and (2) convince users it was safe and not a keylogger when they've been told a million times never to run exectuables in addons.

How often are users really updating the compilation, anyway? Users should be updating the individual addons using the Curse Client or MMO Minion (or doing it by hand); you shouldn't be uploading a 25MB+ package and having hundreds of users download a 25MB+ package and reinstall their whole UI every time any addon is updated. If you're using custom code, you should consider having two downloads: the base package with the WTF folder, and the "update" package that only includes the code updates. Users who already have your compilation installed should use the update package; only new users should download the whole thing.