With SDR technology any standard is possible. Its only a reference point.
50 microvolts for S9 is fine with 3db accuracy for general purpose use. It would at least give enough accuracy for everyday ham activities.
The DBuv scale is more useful and accurate. A default back to Dbm would also be useful. Again all possible on the SDR platform.
A more important calibration scale these days would a 9khz EMC defined bandwidth with a Quasi Peak or Average DbUV calibration scale.
This would be very useful for hams for quick on the run radiated emissions measurements. If we used say a 1 meter loop with a known Antenna Factor we could document and report all these lousy crap equipment that is causing hams so much trouble these days.
SDR radios can deliver all these S-meter calibration scales that you want for no extra costs.
Regardless of the calibration scale, we need any accuracy reference, its certainly better than none.
Which S meter calibration levels would you use. There are many standards beside 50uV at S9. At the lower scale how many dB from S1 to S3? 2 dB or 3 dB?
Unless you take the mic. gain control out of the radio and hard wire the microphone the IMD is not worth worrying about.