It pains me to say it but AutoCAD has a leg up on Revit when it comes to keynotes.
In Revit, the tab-delimited text file just does not cut it. It is too cumbersome to edit, unsearchable from within Revit, lacks organization in the keynote legend...
AutoCAD uses a proper database and its keynote legends are beautifully organized by division and section. The database is also searchable when placing a keynote.
I think Revit should mimic AutoCADs keynote system and then use its current keynote system to replace the note block. Then people could create a tab-delimited file to house all of their sheet notes (though this needs to be editable within Revit) with the ability to filter by sheet (which is in high demand) and they could use the new awesome keynote database for their keynotes.