Great idea, here are some comments
1. We really should find something to eliminate the need for an extra query and an xml parse for each page view. Large sites cannot afford this. Of course smaller sites can use this, but hack authors would still require filed edits, so their hacks don't slow down large sites.
2. Since there are file edits anyway, why not edit global.php instead. I think some re-arrangement can help, such as moving init_language to below a hook (can't see any side effects, but I could be wrong). This, I assume will solve the phrasegroup issue?
3. What's left are templates. But since we can already modify global templates with a plugin, and if it uses an if statement to check for the script being used before merging the hack's template, then we're done (except for special templates)
But, let's hope vB Dev's surprise us before gold, and we don't have to mock things up

I noticed Scott's reply to Kirbey's thread, so I am optimistic.