Meredith Monk sometimes writes in gibberish.

I recently came by two CDs of music from the Damanhur neo-pagan religion, which is ritual and mythological texts in a language they made up. They're at http://www.damanhur.info or http://www.damanhur.org, but no samples of the music that I can see - it's okay but not all that distinctive.

Ted Hughes's "Orghast" was probably the highest-profile effort along these lines, funded by the Shah of Iran for a performance intended to attract an international jet-set audience. I now wish I'd bought the book when I had the chance, it's probably a collector's piece now.