@DoubleMIDI Yes, you are correct about the difference in time and phase delay. I absolutely understand it very well.
I don't think variable phase is a silver bullet in this case where both pickups are identical and the propagation delay through the bridge is relative to the respective distance from each string to either pickup. However, a phase shift of one pickup relative to the other will result in shifting the comb filter effect and may shift it into a much more pleasing and usable response.
To verify this, running each pickup into different EQ settings would effectively change the phase of each pickup and change how they interact with each other. Each band of EQ will introduce it's own phase delay, so it would be very easy to get a sense of how phase affects the sound. Of course, you have to listen to a different frequency band than you are adjusting (turn down the lows on one pickup, listen to the mid-range and highs to see if something else shifts, etc).
A number of commercially available Piezos use multiple elements within their singular packaging. They are placed much closer to each other than the the Yamahiko, but there are often more than one. The Realist and FC both come to mind. They might not be your favorite, but they've worked for many people for many years.
