Just wrapping this up with some interesting observations.
In retrospect getting the plain vanilla P232 was a big mistake. I should have put the extra money on the table for the Spectraview (EU) edition. The unit itself is a great little monitor, but calibrating it turned out to be a lot trickier than I had anticipated.
Recalibrating with i1 Profiler turned up the dreaded color banding again and in fact I have not been able to eliminate it entirely since. I've also tried ColorEyes Display Pro, using both the i1 Display sensor and a Spyder 3 sensor. I've tried calibrating at different precision levels, but actually the higher the precision the worse the banding (makes sense since the curves get more quirky). In some cases it has been so pronounced that it would probably show up in images with me trying to "correct" it.
So to all calibrator-makers out there: Leave my videocard alone. If you can't get to the monitor's internal LUT to do a calibration there, don't do it at all. I've been to the ColorEyes forum to ask Jack Bingham about this, could we have a no-calibration/profile-only option, but he didn't believe this could ever be a problem, so no, forget it.
Anyway, I decided to give ArgyllCMS/DispcalGUI a try, because I saw that it does have just such a no calibration/profiling only option. So what I do now is to set the white point luminance, temperature, and black point in the monitor's OSD. Confirm with sensor reading (using any calibrator that has a precalibration function) - or I'm sure you could even do this visually if you're confident and experienced. The main thing is to get that paper white and appropriate black level/contrast.
Then run "Profiling only" in Dispcal, making sure you have individual RGB curves to take care of any color cast. The result is beautiful (at least it is with this monitor). No more banding.
I've tested this pretty extensively by now against my work setup, a hardware calibrated Eizo. I'm simply unable to see any discrepancies. AFAICT everything now displays identically on the two systems, except of course that the Eizo is wide gamut and the NEC not. Certain reds even display more consistently with this setup than they did when running a full i1 calibration/profiling on the NEC.
Obviously this only works as intended with color managed software, and the rest be damned. But that's fine with me. The P232 shines, and proves what a great little monitor this really is. The only thing about it that I'm not too happy with is that the off-angle IPS white glow is pretty pronounced, but in actual practice it's not a problem.