Here's another interesting discussion:
FLO Cycling - Why Do You Use Less Tire Pressure for a Bigger Tire or Wider Wheel?
He talks about how rim width affects casing tension, but I don't see where he talks about the stress that casing tension puts on the rim, and how that changes with rim width. The casing and bead are pulling the rim in a particular direction. A narrow tire will tend to pull the rim away from the hub. On the same rim, a wider tire is going to pull more and more in line with the axis - the two sides of the rim are getting pulled in opposite directions.
This is all about the maximum safe pressure for the tire, rather than the optimum pressure for efficiency. For efficiency, it does seem like a trade-off between rolling resistance and suspension. There is energy lost when a bumpy ride shakes the bike. If you mostly stand on the pedals then the bumps don't shake you much. Do you have luggage on the bike that is getting shaken and absorbing energy? The most efficient tire pressure depends on all these factors!