There is pretty ample information available for most custom Commodore chips. For some chips, like the PLA on the C64, there is no datasheet available, but for these there was some reverse engineering efforts.

One custom CSG chip I cannot find any information about is the Disk Controller Chip for the 1541 disk drive, the MOS 325572-01.

Does anyone perhaps have some interesting links regarding the internals of this chip?

I imagine there'll be documentation, reverse-engineered or ottherwise, floating around C64 emulator and demoscene circles. It was very common to install fastloaders into the 1541, which had its own 6502, at runtime - and those have to be correctly emulated to run both classic games and modern demos.

1540\1541 Service Manual, preliminary, Apr. 1985, PDF page 35 shows a TTL equivalent of the MOS 325572 gate array.
Also, the PDF contains schematics for PCB #1540049 (1541 board with MOS 325572) and PCB #1540008 (older 1541 board which uses TTL chips instead of the MOS 325572).
Hope, this helps.

Thanks for the info @ttlworks. This was exactly what I was looking for!

Nice that so much of it is defined in terms of 74-series equivalents. You could directly build a substitute on a daughtercard if you had to.

Earlier versions of the 1541 did use 74-series chips for these functions, and I expect that equivalent schematic was redrawn directly from the original. You can find that on page 15 of http://www.zimmers.net/anonftp/pub/cbm/schematics/drives/new/1541/1540-1541_Disk_Drive_Service_Manual_Preliminary_314002-01_(1985_Apr).pdf