When I looked for Nivers some time ago, I found quite a few of the chant books he edited at Google Books.
An oratorian chant handbook (containing, among other things, an early version of today's "tonus simplex" of Salve Regina) can be found at Gallica, more stuff could probably be dug up there.
Two examples here! You're right, in context, on reflection, they look like sharps. The little + looks like a trill though. I wonder if chant in this period would have been accompanied.
The +'s are trills and the x's are indeed sharps. This chant was sung much as non-chant sacred music or secular songs would have been sung, with appropriate ornamentation, rhythmicisation, and shamelessly 'tonalised' modality. A prime concern was modernity and erasure of the 'barbaric' mediaeval aesthetic.
I used to sing with a French group (based in Bordeaux) called Les Chantres de Saint Hilaire which specialised in this sort of stuff. I seem to remember a disc of Eustache du Caurroy we did had a couple of tracks of chant with serpent. It was a while ago & I don't think the disc got a very wide distribution & I no longer have my copy (nor do I any longer live in France,sadly) although I think I may have it somewhere on file. I shall look it out. As you correctly say, it has a lot of ornamentation.
To participate in the discussions on Catholic church music, sign in or register as a forum member, The forum is a project of the Church Music Association of America.