I think a reader of 5-6" sizes aren't really conducive to being used as a ref-doc reader. Whereas you can read the files, most ref-docs are pdf, are are generally suited to a larger-screened reader.
I can see implementing a T9 system into the reader, they way a lot of phones are. It wouldn't be too terribly hard to implement. I can't recall if OpenInkPot supports this, or if it's in plans, but I recall something about an alternate firmware might be working on it. Someone more knowledgeable could verify this though.