I use 2-4" thick White Oak (all heartwood, no sapwood) sills with "Sill Seal" between the White Oak and the conventional masonry.

I understand your comments regarding ring density, but I am not certain if ring density in and of itself affects rot resistance.

Of course if you have a really dense piece of wood there will not be much sapwood, which rots easily.
-Collin

"The lyf so short, the craft so long to lerne."
Geoffrey Chaucer (1343-1400)