Not sure what kind of mill you have, but I know from personal experience that sending flaked grains through the Barley Crusher is a bad idea. I accidentally tossed the flaked barley in with the rest of the pre-crush grist one day, and so I had no choice but to send it through. About 1/3 of the way through, the slave roller stopped spinning and thus crushing activity came to a halt. After messing with it for a few minutes (I was sure my BC was f*cked), I found that some flakes had gotten stuck between the slave roller and the sheet metal fin that directs grains into the roller gap. It's easily solved by lifting up the whole unit and spinning the slave roller backwards by hand to unstick the jam...but it gets to be a PITA when it keeps happening every 30 seconds.
As such, at least with the BC, I'd advise against it.