Maybe its a copyright thing? The varied hymns drive me crazy too. "All Glory, Praise, and Honor, To You Redeemer King" just doesn't sound right.
I'm not familiar enough with the english Nervous Disorder plainchant masses, although I have heard of the two you mention.
Aren't there plainchant settings in English in the 1982 Hymnal? Why not use those?
We're doing Gregorian right now. I hope to interest our choirmaster in the four- and six- part Anglican chant sooner or later - all in good time. There's one for Good Friday (setting of Ps. 22) that is just absolutely incredible - I'm starting to work on the choirmaster NOW for next year.