MBP mid-2012 with SSD, does not hibernate
I've the same problem using late 2008 Macbook Pro running 10.8.3 and have decided to keep the HDD in the shock mounted area due to superior protection.

I've got a Sandisk SSD installed in the optibay and the original HD in the main bay. Deep sleep worked fine with just the hard disk installed. Now that I've cloned it and boot from the SSD instead, the Macbook refuses to go into deep sleep. Instead it sits there in normal sleep mode with the white light on the front of the machine flashing.

I tried moving the sleep image to the HD but got the same crash that you experienced. Mind you, the man page for pmset does state that it has to be on the root drive!

I'm reluctant to swap the disks round because the main bay offers better shock protection for hard disks with the rubber mounts. Looks like I'm stuck with the problem unless Apple fix things.

As it stands, you need to choose either sleep compatibility or shock protection when connecting a SSD and HDD to a MacBook Pro.