Is it possible to create an out-of-phase pickup sound digitally, and has anyone done it? I was thinking maybe I could run two parametric EQs in parallel in my H90?
Yes. It involves, ...get this... a phaser. But you need one where you can set the modulation to 0 and set the sweep to a particular spot to get the sound you want. Wouldn't be surprised if your H9 had something built-in.
Just slight correction, not 'sweep' but you need to set the delay time (on the order of single-digit milliseconds) to get the right out of phase sound you want. It's essentially making your signal a two-pickup signal and putting one of them out of phase.