ChrisF I’ve made a lot of brats from just chicken breast and the best results I had were when I used a combination of Cold Phosphate and Carrot Fiber or Super Bind. The Phosphate increases the water holding capacity of the meat and will super bind you get additional water holding capacity form the Carrot Fiber and the potato starch will gel up at about the same temperature that the meat will start to sweat at, so it will absorb it as the meat sweats it out.
Now, nothing is going to work as well as pork fat, at least not in my mind. Here are a couple of videos/articles we did on this in the past:
https://meatgistics.waltonsinc.com/topic/415/how-to-make-a-juicy-homemade-turkey-bratwurst/
https://meatgistics.waltonsinc.com/topic/264/how-to-make-homemade-chicken-brats-recipe/
https://meatgistics.waltonsinc.com/topic/393/how-to-make-juicier-chicken-brats-recipe