Nick,
For KCBS, the smoke ring means absolutely nothing !!! We judges can not take a smoke ring into account when scoring since the smoke ring can be create artificially.
With the above in mind, the brisket surface need to meet certain criteria (if you will...) before it will accept the oxides of nitrogen. When trimming, do you also remove the silver skin ? If you coat the brisket with something liquid when applying rub, make sure it is not a fat nor oil. Brisket needs to be 'wet' to absorb the oxides of nitrogen. Keeping the meat as cold as possible will lengthen the time the nitrogen oxides can do their stuff... will increase the size, thickness, of the ring.
If you are of the assumption that without a smoke ring, your brisket will not do well... then you are mistaken. First and foremost... Cook it properly and to perfect (if there is such a thing) tenderness. Everything else will fall in line...