Grimrock

You can easily use Sysinternals (now Microsoft's) Procmon with a filter of "Process Name" grimlock.exe, Operation is CreateFile and Path begins with "C:\Program Files..." (whatever your path is) to see what files your Grimrock tried to open to override files from the GRAR. The on...

I haven't seen anyone mention this, but the file format you're all pulling elements out of is documented at https://github.com/n2liquid/GRiFS/wiki. There is C++ code on there to interact with the virtual filesystem they're using. Also looks pretty promising to be able to make new GRARs that will act...