I found a soy flour in the store yesterday that is a very fine dust. Can this be used in sausage recipes that call for soy protein?
Sure, but It wouldn't be substituted 1 for 1. Just looked around quickly and found that soy protein concentrate is about 60% protein and the flour is about 40%.
There's a lot of people that use flours for binders including tapioca, rice, wheat and oat to name a few.
As said, soy protein concentrates work well. Different types might have different flavor profiles. Non fat dried skim milk powder can be used as well, but again will be slightly different.
Good luck and good smoking.