I typically induce feedback with digital amps (Fractal, VSTs, etc.) one of two ways--a cranked FRFR or an EBow. The EBow method is tricky--requires forethought in getting it prepped for use when both hands are occupied, but I tend to like the results better as I can pretty much guarantee the feedback with be a harmonic of the note I'm playing. With a cranked studio monitor or FRFR, the feedback is obviously more "natural", but harder to control.
Given that I pretty much never play live, I have the luxury of combining both methods when I'm recording (provided my wife is ok with the racket!). That tends to sound best, as I can get the string moving at the right pitch with the EBow, then sustain it into pleasing feedback with the FRFR.