Would be nice if you could just wipe the MFT's free space by itself. Sounds like the only way to do it currently is a wipe free space on the drive with the MFT option checked, but then you have to cancel the process once it gets past the MFT part.
See below:
http://forum.piriform.com/index.php?s=6b104f44393780c49cecfd0ffd1ad395&showtopic=45513