After someone mentioned to me that there were a few CLZ files embedded in the PS2 version of AWL, I decided to take a look. Upon extrating the game data, I found they were right.
Not only that, but it appears one of the files (mainchapter0) exists in both arc and arc.clz format; possibly leftovers from porting the gamecube assets. This is nice as it provides a start and end point to look at for the compression.
Right now my working theory is that the clz files are non-standard single-file archives with some form of Lempel–Ziv compression (e.g. LZW, LZ77, LZH, etc).
It’ll take more testing to see if my theory holds and I can find some way of uncompressing and recompressing the files.