I've used 5 inch aerial colour neg in my camera with no problems. The film had an orange mask the same as regular C41, I think it was Agfa but would have to check.

Keith thanks for the posting the emails here, yes I dealt with the same guy. I no longer have my email exchange with him. One thing I'd be concerned about is getting the film plane position correct for the holders. Did you send them a holder? My 5x12 was built for S&S holders which they had a set of. The 4x10 holders I have are Canham, do you know what holders your camera was set up for?