I think it's in the liber cantualis, If I recall. I can scan it tonight or tomorrow, if you're interested.

That may well be true. I am not here to help only to note two things.

I do know somewhere in the middle ages the hymn ubi caritas sometimes began with different verses than it does now, not recalling the details, that doesnt mean I can answer your question.

However, the basic question you raise is tremendously interesting.

What I do know from experience is that some of the most important hymns of the office or mass processions are excerpts of much longer hymns. Generally the earlier in history, the more verses are encountered. Typically after the Council of trent, besides entire revision elimination of the words themselves, many verses were also removed, making for instance the "Aurea luce et decore roseo" (SS. Peter & Paul) go from 7 verses before circa 1550 to become only 4 verses after about that time.

Sometimes the verses that were lost are tremendously rich and interesting.

An excellent example of this, which i probably bring up far too often, is "Of the Father's Love (heart) begotten" . This is one of the greatest latin hymns of the church written by Aurelius Prudentius (4th c.) It was popularized by Anglicans in the oxford movement and by now is sung in almost every christian denomination during christmas. This is quite a feat, the modernists, the unitarian temple, SSPX chapel, everyone sings this song (at least in english) and it is enjoyed by all. It was also sung in the York use at compline for christmas for centuries in latin (though to a different melody as used today.)

Anyway, the lines "Of the Fathers Love Begotten "Corde natus ex parentis" sung today, are but an excerpt of a much longer hymn. Of all the hymns I have seen, this is one of the most profound and meaningful I have ever sung in the Latin church.

What is terrific is that the rest of the verses of the hymn can be sung at Ascension and Pascha too, and arent probably fitting for Nativity. So it was a goldmine for me to discover.

"http://www.ccel.org/ccel/prudentius/cathimerinon.p09t.html" (english)
"http://www.ccel.org/ccel/prudentius/cathimerinon.p09o.html" (latin)

Most of that metrical translation is very accurate of what the latin says as well.
Interesting, translating latin hymns of trochaic tetrameter into english is much easier than say, iambic dimeter, the most common meter. with Trochaic one can be assured of much more accurate english metrical translations, why that is is best explained by someone else.

So keep up the work at finding this marvelous buried treasure of ancient latin hymnody. It is such fun to imagine and realize the creative ways it can be sung in churches today.

"Christ, our Captain, for a season deigned to dwell in Death's domain,
That the dead, long time imprisoned, might return to life again,
Breaking by His great example ancient sins' enthralling chain."

Who wouldn't want to sing words like that? Preaching to the choir I am eh?
;-)