I agree Matthew, I also tend to use the primary K-edge peak for P calibration, but one issue to be wary of is attenuation/flattening of the primary peak (if one is using a concentrated sample).
Gypsum sounds like a good material to use for S, since it is commonly available and probably not too variable. My material of choice for P (lazulite) might fail on both counts, so it might be a poor choice.
We're always on the lookout for better sulfur standards. We find scotch tape (magic tape that is), actually has pretty reproducible XANES and is easy to come by. Double-sided tape seems slightly different.
We tend to find gypsum slightly unreliable as a sulfur standard as there is some variation in samples. Attached is a plot of 6 different gypsum samples we've run (all in fluorescence), from a variety of user groups over a few years. You can see that there are differences in the low energy peaks and large variations in the main sulfate peak intensity. But the energy position is stable. Some of the variations may be due to over-absorption, but the low-energy peaks suggest that "gypsum" is sometimes not high purity.
As I alluded to earlier, we never calibrate the energy at S, we just measure it. We regularly check Fe and Cu. These rarely vary by more than 0.5 eV unless we have "calibrated" to a user-preferred-but-we-know-to-be-incorrect value, and we believe we have the lattice constant pretty good to reproduce the Kraft values. The gypsum peak positions are pretty reliable and are definitely well below 2482 eV, more like 2481.75 eV.
For sure, we would be very interested in trying a Bond spectrometer like Stumpfel et al user (or the clever Bond-like approach that Pettifer and Hermes used before them). We don't have a lot of room in our station for a large diffractometer, but we do have access to area detectors these days, so it is possible that multiple simultaneous reflections from fine Si powder could be used.
--Matt