Is there a working way to request explicit huge pages for a file-backed mmap on ext4/xfs/btrfs? I'm not aware of it.

I have no idea. I was pointing out that the transparent huge pages documentation is not authorative for requesting huge pages explicitly with mmap.

I didn't find any documentation that would indicate that explicit huge pages didn't work with on-disk filesystems, but sure enough, it doesn't seem to work on ext4.