it should just work if it's an ISO
Discussion
Seems to get stuck on “Waiting for system partition to become available”. 6.7 fwiw
“Failure: Reached timeout waiting for system partition. Not restoring random seed from LBA 34.”
Made some progress after that, I think. Loaded squashfs anyway.
maybe try adding "toram" to the kernel parameters (press e on the menu item) - that can be hit and miss but in theory it copies the whole squashfs to a ramdisk, so obviously you need to have a few GB of memory
Spoke too soon. It’s looking booty now. 
“[FAILED] Failed to start systemd-journald.service - Journal Service.”
Now in a loop of this problem. Tails seems to always do this to me as of version 6+. Older versions don’t seem to have this problem.
But at least I’ve confirmed that the iso boots from Ventoy.
yeah, probably fucking linux kernel, which has been a tire fire since version 6