I like the idea, but the de-Unicodifier would have to use the current user's locale - de-Unicodifying certain characters (e.g. Cyrillic that look like English) would have unintended consequences if you just assumed they're meant to be English.
My idea was that this would be something provided manually by the users, just like you can specify alt text for images on Twitter right now. I know many wouldn't bother. But at least some would, and it would make using an upside down nickname possible for me again. :)
The hosting service can solve this million times as effectively as the users can.
That wouldn't work either. What if I was born with a foreign name, but was raised in a totally different country?
It should be easy to detect. You usually don't mix different alphabets in a single word.