I recntly inquired with a guitar shop on the possible installation of a Sustainiac in my JTV-69.  Here is their reply and attached pictures.   I'm assuming the Fernandes installation would be very similiar.  Just passing this on, as I know many would love to add this capability.   Not worth the tradeoffs for me at this time...hopefully L6 will build this feature in future models.....

"Thanks for your interest in the Sustainiac products.  We have had one of these in the shop for installation.  It is a difficult thing to do, and it “violates” our standard of operation.  Our hookup diagrams are designed so that when you turn the sustainer ON, the bridge pickup is automatically selected.  This is necessary for two reasons.  First, the bridge pickup is furthest from the Sustainiac driver.  If the middle pickup was selected, the sustainer would oscillate (squeal) uncontrollably.  And obviously, the neck pickup (Sustainiac driver) cannot be serving simultaneously as a driver and also a pickup.

But this guitar cannot be done this way because one section of the two-section selector switch is used by the Variax function.  So, if the sustainer is ON and the middle pickup is selected, it will squeal.  Or if the neck pickup is selected, you will hear nothing.  The customer was willing to live with this limitation. 

The only place to put the Sustainiac controls and circuit board is as shown in one of the enclosed photos.  Use of the Variax battery is not practical because it causes adverse interaction between the two systems. 

We charge $150 to do this installation, plus $25 to install the optional $16.00 battery case."