I love love love the Sustainiacs! I was an EBow guy for years, and I took a leap of faith installing one in my Deluxe (I had the Sustainiac folks install it for me), and I love it! I use it all the time. It's like the best controllable feedback at any level, and there are three different sustain "harmonic" modes. Only bad news is they have to put two screws through your instrument:

On my Deluxe, they also removed the Piezo Powerchip and Volume Pot in order to fit it in and utilize the battery cavity. On my Mojo with the Sustainiac, I have the Piezo and the Sustainiac squeezed in there with two battery cavities. The Sustainiac as a neck pickup is also not bad by itself (without sustain turned on).

Anyway, PM me if you have questions. I'm jealous of your Spanish Fly!! I've been looking for one for a long time, and, while I recently bought a Godin Multiac that I'm quite happy with, I still keep a watchful eye for the Nylon Fly!