So, it loads some kind of shim that presents as a BIOS optical drive, but reads its data via NBD? Then, say, the linux kernel of the vanilla ISO installer sees

So, it loads some kind of shim that presents as a BIOS optical drive, but reads its data via NBD? Then, say, the linux kernel of the vanilla ISO installer sees that shim program as a sort of virtual cdrom? When you installed Mint, did you happen to notice from where the installer thought it was reading the files?