I've just looked into this myself, and done a blog post
In your PPD file, insert after the *JCLCloseUI: *JCLBanner line and before the *% Generic Accounting line)…
*% Secure Print
*JCLOpenUI *JCLSecure/Secure Print: Boolean
*OrderDependency: 10.0 JCLSetup *JCLSecure
*JCLSecure False/Off: ""
*JCLSecure True/On: "@PJL COMMENT OID_ATT_JOB_TYPE OID_VAL_JOB_TYPE_SECURE_PRINT;<0A>@PJL COMMENT OID_ATT_JOB_PASSWORD "AYWQ";<0A>"
This should now give you a “Secure Print” tick box on your Advanced tab of your print dialogue.
The AYWQ is the encrypted PIN. I’ve no idea what the encryption method is, I obtained it by capturing a print job sent with the Windows driver. In this case the PIN is 1234. I can’t see any method of being able to choose your own PIN at print time (unless anyone else has any ideas?) so its not terribly secure obviously, but its better than nothing – you’ll at least know that someone has printed and walked off with your job!