Additional Data (generated as response to a question on Facebook):
The fuel filter was replaced last week (shortly after I noticed the symptoms, but before I started testing fuel pressures) and the battery was disconnected for a few hours so that any fuel trims in the ECU would be reset. Although there was a minor improvement (and I suspect it had more to do with resetting the ECU fuel trims than replacing the filter), the behavior is largely the same.
The car is running on the stock ECU at present (1997, CA-emissions, manual transmission). The injectors are stock. The only thing that is non-stock at the moment is the fuel pump. The O2 sensors have been replaced recently as well. At first I put a brand new Bosch sensor in the front position, and then after reading some reports that the Bosch sensors don't always play nice with our cars I moved it to the rear position and installed a known-good slightly-used Denso sensor in the front position. Data from the OBD2 shows no significant change in signal behavior of one sensor vs the other, and wideband O2 confirms that the fueling remains the same as it was too.
I have been running the car on an MS3 in parallel for the past few years as I've been trying to create a tune that so closely replicates the factory tune that it will "play nice" and pass all of the OBD2 emissions readiness monitors and run without causing any CELs. I've been able to get it to run without causing CELs, but I was still having difficulty getting it to pass the self-tests, so I switched it back to the OBD2 for a while to do some comparison testing. The OBD2 is not clearing the self-tests either, but otherwise seems to be running fine. Then after about of week or two of running on the OBD2 computer I noticed the odd lean behavior at WOT. It's possible that it's been that way this entire time and I just didn't notice because I wasn't holding WOT through that range very frequently.
I suppose I might need to hook up the MS3 again and see if it's running leaner than when I disconnected it--which would indicate a hardware issue and not an ECU issue--but if it doesn't run any leaner then I'm still no closer to a solution (as it may have been tuned to compensate for whatever is happening, if whatever's going on has been a problem for a while that just went unnoticed since I could make minor adjustments for it as I tuned).
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