At first the script failed with error messages when trying to extract my hlm2 wad files purchased at gog.com. After some fiddling with the offsets I got it working though; In my case the wad_offset has to start at 16 and after compiling the asset list the wad_offset has to be incremented to skip what seems to be the folder structure. I couldn't find a way to determine that length programmatically.

Fiddled parse.py that works for my music wad file
hlm2_music_desktop_gog_head.wad.gz - Head of my music wad file until the first ogg file

So now I wonder if this is specific to the wad files when purchased from gog.com and if someone knows a way to determine the length of that folder structure block programmatically for arbitrary files.