wimlib

Yes, I know it created the file system because GRUB successfully installed on the raw disk image.

It would be nice if I could use wimapply directly on a raw disk image, instead of mounting it (which is actually a pain).

For example:
wimapply install.wim 1 file.img

Right now, it's displaying this error:

[ERROR] Failed to mount "file.img" with NTFS-3G: Invalid argument
ERROR: Exiting with ...