I don't know the answer for sure, but I can offer some ideas.
If I were getting porous quartzite countertops, I'd have the fabricator put the slab up on edge and seal the heck out of the upward-facing edge, then repeat on all sides. There's a technique called 'puddling in,' where a lot of sealer is put on the surface (like a puddle) and left to sit and soak in for awhile. Obviously the surface needs to be horizontal for this to work, hence the idea of putting the slab up on edge. Also, this would need to be done before it's installed.
Then repeat the same method on the top, so that all the edges and the top are sealed well.
If you have a willing fabricator, it would be useful to test this idea. It's something on my list to try.
Q