All the recipes I’ve seen, which admittedly aren’t that many, tend to use the spent grain as a small portion added to a normal flour base. I too had my “economical” side hoping to find a good use for spent grain when I went AG, but the spent grain breads I saw seemed counterproductive since they didn’t really use spent grain as more than an additive.
Bearing in mind the grain is “spent” in the sense that all its sugars and fermentables get washed away leaving husk and mashed/rinsed malt, even if it could be processed into flour it might not really produce a nice tasting bread. I dunno though!