I've been trying to figure out these things for a good while now to no avail. All of the games assets are stored within these TPK files; they appear to be nothing more than a container though. I did see the bms script for Senren Kagura via a search, but that sadly didn't work.
Here's some screenshots and that - it's worth noting the game's music is made up of loops, hence the small filesizes.
I'm mostly trying to get the music loops outta there more than anything else (attempting to build an unofficial OST, natch) though if someone can figure out what format the graphics are stored in too, that'd be real cool.
Note for anyone else visiting here later; The graphics get extracted as regular .GIM files, while the audio gets extracted as *.DAT and *.VAG files - you can use PSound on the latter to convert it into *.wav~
