Progress! Thanks to a big help from a fine person at Zenhax, we managed to get a script that extracts the contents of the archive, and the underlying archives within. Most seem like semi-standard psp formats. However, the files I believe are models, come in series of files with numerically organized filenames. The file type is CPK, but all research into that has not been able to extract these. Looking briefly into it hex-wise I find the header is 'CPK0' with some of them also having a second header of 'PTEX'.
Sample Files
Sample Files