Does the same pronunciation hold for Agnus? If so, are there northern manuscripts for the Agnus Dei marked in such a way that would shed light on this question?

IIRC, Dom Saulnier asserted that there was no need for the chants of the Ordinary to be notated in the first place, as there were in most cases a handful of them regionally used, and committed to memory by local communities. (Mass VIII is an exception, a much later addition.)

So while the manuscripts of the ordinaries date from a later time, it would not usually follow that they were composed later.

The propers represent a much larger body of music, and were notated earlier out of necessity.

Your speculation about the northern pronunciation of a harder g may be on the right track. Interesting.

If this is a "nerdy" thread, let me be the first to congratulate you for starting it, as I am new to the world of chant and need a lot of "nerds" to help me through. With regard to pronunciations, I'm also having trouble with my choir differentiating between Germanic and Ecclesiastical Latin. I also dare say, that most Americans are going to have difficulty in pronouncing some of the Latin correctly simply because we all speak conversational English. In any case, please continue with the "nerdiness" because I, for one, need it!

cgz, that's how I would observe it, too.
The Germanic pronunciation idea is interesting, and its worth investigating other ideas, too.

Still, were I singing this chant next week, I'd prepare it as cgz indicates. (If only I could figure out how to do the IPA characters on my iPhone...)

I sing in seven languages, mostly Romance languages, and that approach would be based on my experience and intuition. It's doable and sensible. But of course I could be mistaken.

Fwiw, what I would want if I were leading your schola would be a decision and singing through the whole chant until its comfortable, beautiful, almost memorized. Detailed questions can take precious rehearsal time off-course, so I'd refer discussion to break times and focus on making it beautiful during singing rehearsal. Though I have to say I love it when the singers I direct care enough to get into the details- shows their interest level. :)

Liquescents are used inconsistently in the manuscripts (generally as a reminder) and transcribed inconsistently from the manuscripts to the Vatican edition. And they refer to a Latin pronunciation long before it was standardized.

Although being able to identify liquescents is important in determining the rhythm of the medieval practice, one can obtain a satisfactory performance in the modern 19th century style by ignoring them altogether.

Thanks to all who joined this conversation. My question regarding the liquescent clivis on "RE-gnum" in the Communio "Primum quaerite" could of course be approached from at least two perspectives:
1) an academic-historical perspective: how was this word pronounced in the 8-9th c. and was that particular pronunciation the reason for the liquescent here and in similar words having "gn" between two vowels?
2) a practical performance perspective: given that our schola follows the pronunciation rules of the Roman liturgy, how will we sing it at Sunday's mass?

My original posting was really seeking an answer from perspective 1). Thanks to Ioannes Andreades for helpful information.

As for 2), what we did on Sunday was to sing the clivis on the open syllable "Re-" and to treat the "gn" sound as the opening consonant of the second syllable of the word. That solution of course turns the liquescent second note of the clivis into a normal note but has the advantage that it is easier to sing: the singers sing on the vowel without getting hung up in humming consonants. (In other situations, to be sure, the liquescent would be all but indispensable for a clear articulation of the text.)