The Eoxian battleship, Thaumtech Omenbringer, has biometric locks for security.
At first, I did a double take, biometrics for undead? They don't have any, right?
The definition of the system just says it can only be used by certain creatures as designated.
So, just what are the "bio"-metrics for a type of creature where most examples no longer have a working biology or even any flesh left?
Do you know how the vast majority of facial recognition software works? By calculating the distance between facial features, like eyes (or eye orbitals in this case) or the nose (or nose cavity, in this case). There's no problem here.
OR, if you want to make the doors essentially unhackable by living creatures, the machine could test their bone marrow.