It's slightly older source, but the shred functionality herein still applies to the latest releases. Look at
https://www-secure.symantec.com/connect/downloads/symantec-pgp-desktop-peer-review-source-code
You have to read and agree to the associated license, but I believe it applies in your case:
1. SOFTWARE PROGRAMS & PURPOSE
You may download the software files made available with this license (“Software”) for the sole and limited purpose of assessing the security features of the Software and verifying that there are no unknown vulnerabilities or bugs in the Software (“Purpose”).
In particular, you may want to look at pgpWipePattern.c
Another thing to consider is to use Whole Disk Encryption to encrypt the drive, and then throw the passphrase away. This is similar to the "I've lost my laptop full of confidential information" use case, which is the primary sales driver for WDE.
Regards,