http://gizmodo.com/5985149/paul-vo-has-made-an-entirely-new-and-impossibly-awesome-instrument
 
Fair warning, there is Stairway to Heaven at the end.

That was interesting.  The part at around 3:17 didn't sound as heavily synthesized as the other sections; suggesting that the effect can be subtle.

Very much like the Psion synth used by John McLaughlin

Terry, do you have a link; I can't find anything about a Psion guitar synthesizer.

Dave, this thread has got me searching and learning (sorry for the tangent Brian)...I found this on gearslutz:
 
 
 
 
Back in the 1970's the Mahavisnu era me and Rex Bogue built a few cool goodies for him
 including circuits installed in the Gibson Doubleneck, the custom Double Penis Rex Bogue doubleneck guitar,
 a custom 6 channel guitar synth using 6 Oberheim expander modules, effects, etc.  
 
Not sure if thats the synth terry referred to,
I'm still a noob when it comes to John M.'s works.
 
 
...on the hunt,
elwood