I started work this week on custom keywords. This feature is currently implemented rather neatly by woelpad's Greasemonkey script called Substitute Keywords.

The plan is to allow import / export of CSV style lists (frame number, custom keyword) and also allow to edit the keyword directly on the Study page like Woelpad's script. I'm thinking of using a text area to import (similar to Manage > Add Custom Flashcard Selection), as this would allow to paste a complete list of keywords but also to edit just a few keywords at a time.