I was about to file a similar wishlist and found that
there was already a well-evolved set of scripts in this one.
Rather than rely on partition types (luks is a partition type?), I had
my init-premount script use cryptsetup and an encrypted= cmdline, as
follows:
export ENCRYPTED=
for x in $(cat /proc/cmdline); do
case $x in
encrypted=*)
ENCRYPTED=${x#encrypted=}
if cryptsetup isLuks ${ENCRYPTED}
then
cryptsetup luksOpen ${ENCRYPTED} ${ROOT#/dev/mapper/}
fi
;;
esac
done
