The files created in Fresco always open for me with the last brush used, which is great, but any other brushes I used in that file are just a mystery. I do have favorite brushes set, but that is not much help—those are the same for all files, and I have too many. Can one save brushes used in just that file? The way it works for colors?
Unfortunately at this time we can't save brush settings at all. I like the idea of being able to save them for a specific project as well. It might be helpful to post this suggestion in the Adobe Fresco UserVoice pages.
At the moment, there's no way to save the brush. Unless its one of the Live brushes or the default pen or pencil and you have the Desktop Photoshop, you might be able to build the brush in PS and save it to your library and then be able to use it in Fresco.