Comment # 1 on bug 1137056 from Andreas Stieger

This has been frequently requested. But please be aware that the security
drawback is that in the running system, the keyfile would be readable in plain
from /boot. Not a good idea.

The better solution is the implementation in grub that allows it to pass the
passphrase to the booted system without it appearing in plain in the kernel
command line or the logs.

This request should be closed.