In case anyone might be interested, this project: http://musmed.eu/

is a long term project to index the medieval manuscripts which are online (in the form of scans), creating a listing of all the antiphons, tropes, and other chants therein, and you can then look at the image of the page where a particular chant is to see that part of the manuscript. As far as I understand it is the volunteer labor of various medieval studies and musicology folks in Europe.

There's an associated blog and forum here: http://gregorian-chant.ning.com/

Not always in English, but automatic translators are good for that usually.