I really like the thought of having things in the header editable... didn’t even consider that... SEO meta tags, keywords and description heaven!!! Could even make it so that they float above the page visible like you said... a nice transparent box... hmmmm... yummy!
I still think this would be a great plugin, implementation wise. But if it will be easier to incorporate as a core modification, then so be it. I’ll defer to your collective judgments there.
A plugin would be, from a marketing standpoint, a powerful testament to the flexibility and robustness of the core. That’s my primary reason for wanting it that way. Even if the core had to be tweaked a bit to allow it to happen...
Whatever the implmentation, it has to be dead easy for users.