I emailed Alan Hoover of Mainiac Music on this (very helpful), and his experience was the Variax output had phasing issues that caused weak Sustainiac response. For  the Sustainiac to work correctly, it needs to be fed a signal from a magnetic bridge pickup. The cool thing (at least, for me) is the Sustainiac and bridge pickup don't need to be connected to the Variax electronics or the output jack in any way; they're just there to vibrate the strings, so that any vax model can sustain infinitely at the flick of a switch.

Of course, if you have an expensive bridge pup and you actually want to access its tones, this too can be done, but the install becomes more complicated. In my opinion, if you're going that route, you might as well get a JTV.

Anyway, hope that helped some people who've been planning a Sustainiac install.