There is another option from the guys who make the sustainiac http://www.sustainiac.com/model-b.htm but I have never heard of anybody using it (and I doubt that you will be so inclined)
I have one!
I am starting to use it as part of my guitar aging /settling in rig that I have discussed before.
It is fun but not so good to use on stage as you need a long cable running from the pedal to the headstock of the guitar. Makes some monstrous sounds though. Sounds like someone has clamped a loudspeaker to the headstock of your guitar.
George Lynch used one quite a bit on Back for the Attack (Dokken - 1987)
I have also started stocking the Transducer version of the Sustainiac although havent statted fitting them yet.
You cant fit them as the middle pickup - has to be in the neck position and it prefers 22fret to 24 fret guitars as the pickup that is acting as the source has to be as far away from the transducer as possible to avoid interference which is why it switches to the bridge pickup when you turn the sustainer on.