I'd have to agree with Patrick. Poking into the DOM and adding the
autocomplete attribute is clean enough for the sort of thing you are
doing.
I look at it this way.. the markup is what the web server sends. The
DOM gives us hooks into the document once its loaded into the browsers
memory. I'm sure a lot of Kiosk extensions for Firefox do this sort of
thing, does that make our markup any less valid?
CB