As usual, it's really hard to find what do exactly systemd. I pretty that we can do the exact same things.
Logically, anything declared at /etc/crypttab should be mounted by the /usr/lib/66/script/crypt.awk script without the need to specify it at the /etc/fstab.
The command of the devices-crypttab service launched at boot is basically:
66-olexec awk -f /usr/lib/66/script/crypt.awk /etc/crypttab
66-olexec ensure that the prompt of the tty used to pass the password is blocked and not visible/used by any other process.
So, if the mountpoint is define at /etc/crypttab it should be unencrypted. After this point, you just need to mount the partition. To be honest i'm not accustomed with device encryption. Maybe i don't understand correctly how the things goes.