I have not hunted in really heavy rains...if it's raining really heavy my experience is there is little deer movement anyway...but I've hunted in fog, mist, light drizzle, and have been caught in a couple of short showers.
IMHO, I think cows knees are an interesting item but for me personally I don't have faith that they seal tight enough to keep out the damp air...ie: it doesn't have to be rain drops to affect things...heavy, wet, damp air will do it...plus, the cow's knee is just another thing to have to deal with before taking a shot.
I also don't like the idea of putting foreign material on moving parts like the pan & frizzen.
I've settled on a few simple things that have kept my lock dry in the above conditions I hunt in and I can bring the rifle into action immediately:
1) I keep the muzzle down at all times;
2) I keep about 18 inches of the barrel/breech/lock/wrist/buttstock area well up under my coat/arm next to my warm body...similar to the purpose of a cow's knee only better;
3) I refresh my prime every half hour;
Just my particular approach to it...