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.

Can you post a link to some example?

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.

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.