Oscar, if someone is using air to fill their tire, and they are seeing a smaller post-cal than their pre-cal (which is almost always the case), then switching to nitrogen with less permeation will cause that post-cal to be even smaller. That's not a good thing.
The end goal, whether with solid tires or with filling tires with something other than air, is to have the cal constant change as little as possible during a measurement. If nitrogen permeates through the tire wall less than oxygen, that would hurt rather than help achieve that end goal. That being said, I doubt the difference in permeability has any measurable effect over the course of a measurement.
Very slow leakage is not necessarily a bad thing. In fact, if you had some kind of pressure regulating valve on your tire that allowed a small amount of leakage to keep the pressure constant as the temperature went up, that would help in keeping the cal constant, constant, and would improve measuring accuracy.