Anyone have experience with any of their brush-on primers?
No, but I had a number of places where, like you, I wanted to just spot-patch an area rather than taking it down to bare metal.
I had decent success spraying a small amount of the zinc-chromate primer into a plastic cup, and then dabbing it on with a small brush.
You may have to do some light sanding to remove any brush-marks, if the areas are in a place where the cosmetics would stick out, but... it works.
$.02
Bruce