I see that you want a good seal at the inside corners of the lands.... so, the bullets shown above would work for your suggested tests, or these as well?... since the powder wraps around the base but not all the way to the center, which is the area I was saying is not important.
I had thought you might be with the "coat the entire base" crowd, but I think see your point. For any improvement in sealing along the sides, it would just need to come completely around and onto the flat part, even a few thousandths would make sure nothing 'whistled past' until after
complete exit, and then the blast would expand uniformly in all directions.
Is that what you were saying? Because coating the center of the base would have no effect on the radial dispersion once the edges of the base clear the crown, that I could see.