I just found a very entertaining book. It turns out that, as with the Bible, a lot of the Latin hymns were glossed in Old English.

However, Old English being a great language for song, it looks (to me, anyway) like some of them got glossed in such manner that you can sing them to the Latin hymn tunes. At least in the Durham ms. :) It looks like they had to just give up on some lines, but others are syllable for syllable. It's possible that quantity or extra notes would explain other word choices, or that elision or drawing out of certain syllables would fudge the rest; but I'm not versed enough on the really old hymn tunes to tell for sure. There doesn't seem to be any attempt to follow standard Old English poetic patterns; but then, people often translate "foreign" or "sacred" material in its style instead of the normal indigenous one.

Anyway, it's called The Latin Hymns of the Anglo-Saxon Church. Very cool. Here it is on Google Books:
http://books.google.com/books?id=c2soAAAAYAAJ&printsec=frontcover&dq="Latin+Hymns+of+the+Anglo-Saxon+Church"&source=bl&ots=ErYQ8EmoLI&sig=6VUvynNz_F3oC_53-LqASe7vPUs&hl=en&ei=SstbS63cLMeY8AaSj5X-BA&sa=X&oi=book_result&ct=result&resnum=1&ved=0CAcQ6AEwAA#v=onepage&q=&f=false

I don't think modern people have been too concerned with the singableness of these glosses, though; it's more been used as a handy glossary for Old English. But I'm not versed enough in academia to know that for sure, either! For all I know, there's an album out from Sequentia or somebody.... :)

It also makes me wonder about whether singable hymn glosses were common back in the day. Obviously, if they were doing that in Old Irish, it would be a cool thing to know. (And hear.) You could have a lot of fun with stuff like that, and it wouldn't surprise me if people did. And interlinear glosses very often seem to have preceded what people think of as the first vernacular translations of stuff... so I'd love to know if this is something people have studied. (Or better, recorded. I like my historical linguistics with a soundtrack.)