R Shaffer, I'll be curious to know how you modify your film holder. I'm currently running some ideas thru my head, but nothing concrete yet. I'm considering that a typical 4x5" holder isn't thick enough to accommodate black velvet, mat board, etc (even by cutting thru the center divider). Perhaps an old Polaroid back would be "cavernous" enough to allow for easier modification?
What type of developer would GP-2 be classified as and how does it differ from the Lumiere Pyro developer? I'm tempted to try the Lumiere developer first, due to its history with the Lippmann process; I feel some assurance that it's "time tested". However, as Bjelkhagen is using the GP-2 with these modern Slavich materials, there's certainly reason to go with that as well.