Tente usar essa entrada em 40_custom
:
menuentry "Tails Security" {
loopback loop (hd0,7)/boot/ISO/tails-i386-1.3.2.iso
linux (loop)/live/vmlinuz boot=live union=aufs noswap noprompt ip=frommedia findiso=/boot/ISO/tails-i386-1.3.2.iso toram=filesystem.squashfs
initrd (loop)/live/initrd.img
}