The chant version looks good, but which line is the do clef supposed to be on? The first staff has the clef on the second line, then it switches between the first and second lines on the remaining staffs....

Interesting and bold to unveil this version, Francis. I think the positive aspect lies in demonstrating that discerning and discriminating guitarists can indeed provide a worthy, underlying accompaniment to a very competent schola chanting. What I notice in the arrangement, though, calls into question the couching of the melody to tonality versus modality, and also to what amount of choral variety and inversions are necessary. This is certainly "ear of the beholder" stuff.
I have occasionally used guitar to accompany ADORO TE or IN PARADISUM, using D as the final. In your example, you seem to clearly use D Major tonality, with the exception of using a II (E) Major at the end of the intonation phrase and elsewhere. I might have used the minor v (Am) in its stead. Also, I tend to opt for an open tonic chord without a third whenever possible, for when you do move diatonically to a IV or V (which I also try to keep 1-5 open if possible) so that the vocal line "mi" isn't overwhelmed by the triadic chord being strummed or arpeggiated, the richness of the third in the IV or V contrasts well. And the D/F# (I first inversion) while the melody hovers around "mi" causes me to wonder if that is over-stressation of the pitch, which could distract from the vocal line. The I 6/4 is less obvious and leads to a transitory cadence on V.
But, this is the kind of thinking that some of us may never have considered valid. So, I'm glad you're sharing your experience. Certainly Julian Bream is smiling somewhere.

Charles (continued)

I dedicate this arrangement to Charles in CenCA who (as you can see above) has hinted to wanting a more authentic harmonization of the chant in keeping with the church mode.

I am glad to see you are coming around to the antiquarian way of thinking, Charles! Congrats!

This harmonization is translated from the organum according to Capella Gregoriana Easo.

Oh, Francis, you have no idea how eclectic I can be! Nicely done.

Actually, Francis, I meant "eclectic" in terms of compositional modalities. And your catalogue far exceeds those products of my muse.
And I've been suffering, literally, with the worst headcold I've had in decades this week, so I can't comply with your challenge.
But I relish the idea of setting a proper or chanted hymn to my chordal vocab, and sharing it with you soon. Will do, absolutely. The "antiquarian" in me wants to somehow, though, set some ars nova like polyphony that utilizes classical guitar (lute) with the cross-relation thirds so bittersweetly set ala Tallis. I've heard ensembles like the Baltimore Consort do such, but never have encountered such used for liturgy.
So, your setting of the "Lapis..." whetted my interest as to whether CMAA folk would give such endeavors a fair audition. I've had young cassocked priests come up to me after hearing a chant accompanied by a decent classical guitar arrangement, or my guitar transcriptions of the Mozart "Ave Verum" or Dubois "Adoramus te Christe," and exclaim "I didn't know you could do that on a guitar!" And that's theoretical kids' stuff, harmonically. I don't think our efforts with guitars, as such, are a pervasive solution to the transitions necessary for RotR. But as you say, they can prove a means to an end.
You know what else would work with your ensemble? Try taking Richard's Simple Choral Gradual and do simple transcriptions of the homophony. Most of them work nicely when transposed via capo up a minor third. Then rolled chords like a Dowland piece. NIce.

Good idea Charles. I will have to look up RRs works. I can't stand playing Now Is The Time To Worship and similar trash. We await something from you when you get a chance. Send over some of your RR transcriptions if you have them.

I am looking forward to your musics. I have some new hymns for guitar that lead the guitar world back to hymnody. I will post soon.

I have never and never will purchase guitar editions. WLPs is a disaster. They set a chord to almost every quarter note.

However, I disagree on that I feel I can arrange anything for guitar. I will attempt your two examples.

I have developed a technique where I can move a flatpick into my palm and back so do both picking and strumming in the same piece.

There is a guitar seminar? Ugh!

The idea is to eliminate guitar from liturgy... Unless it is a liturgy in an extremely small chapel and there is no organ. Then a guitar can work ok if in a pros hands.