Tom, yeah, now that Dave mentioned it, brisket fat in a mix can present you with a seasoning challenge, as the flavor can become strong, if not overwhelming, to some folks. Of course, blending in pork fat will tame it down if the ratio is acceptable. I have used brisket fat in a smoked/dried chicken salami recipe (lean-trimmed dark meat chicken with 17-18% brisket fat, with course meat and crumbed/fine fat grinds) that I developed several years ago, and family and friends rave about it...I literally could not make this recipe often enough to keep everyone happy, and I like it too. Maybe I'm fairly skilled at developing seasoning blends, or maybe I just got lucky and hit the bulls eye right out of the gate on this recipe, but it works well.
Anyway, definitely take the flavor of the brisket fat into consideration, as it won't fit well with existing recipes using shoulder beef fat...it does change the overall flavor profile, and that is where the seasoning challenge begins. The flip-side is that the stronger flavor of the brisket meat, combined with an adequate seasoning blend, may be just the ticket. Probably not something for a typical/traditional style of sausage, but a whole new style you could call your own. I might just be game for it if I had the time to sit down and make it happen.
So, now I probably got you thinkin'...should I go for it, or play it safe?