This is the type of scheme that would be improved hugely by adapting IDF: instead of full copies he'd make the majority incremental or differential, saving time.

I realized today that the mechanism, given multiple devices, where in an IDF one does not always backup from "target" to "drive" but occasionnally from "drive" to "drive" could unlock new possibilities.

Regardless, I would mention IDF to your friend, with the note that he'd have to use backup software that encrypts for it because encrypted LUKS images don't tell their contents and thus can't be used in IDF. This actually leads to the last point about interfaces and tools in my post: it seems that to gain the efficiency of IDF a tool must at least offer encryptation.

Consequently, we get yet another theoretical question: How should the cryptography in IDF-TOH be implemented?

I mean: making F 16 words, D 8 subwords of F, I 4 subwords of F, would hugely improve security!