I think Mont Joye is a stretch also. I found some examples of iridescent glass with cameo and enamel by them, but nothing as textured as this is, or as light in color as the textured portions are. It may remain a mystery.... or someday I will open a book or hit a website doing something else, and it will pop out in front of me.... The style is extremely distinct, and my guess is that the output was quite limited.
Alfredo has the Passau catalogs, I do not (they are on my list for next book purchase), and if something like it was in there my gut feeling is that he would probably recognize it....
It does not strike me as an American piece, although nothing really surprises me with glass any more.