What if the user doesn't have a non-btrfs filesystem? Putting the iso in /boot is the best place because initrd is there and the system won't be accessible if /boot isn't working. People who want btrfs for a root filesystem will just have to put /boot on ext4 or something other than btrfs.
Yep, that sounds somehow quite logical. I'll try to create a boot partition, add it to fstab and see what happens. Hope I do not mess things up