To switch phase you'd need the right type of P90. All the one's I've seen have the single core wiring with a braided screen (that is also the signal ground), so aren't suitable for phase switching. You'd need to find some that had two core wiring plus a separate screen.
For example, the Tonerider and IronGear P90 pickups both have the single core 'vintage braid' wiring.
What pickups did you plan on using?