What I picture as flashing is very thin and wouldn't require much stock removal. If that's the case, I'd use a spiral flute "bridge" reamer and do it by hand. A spiral reamer is different than a drill bit in that, unlike the drill bit, it cuts and pushes the material forward, therefore it is less likely to grab and pull into the material. Trying to set up in a mill or in a lathe is doable. Also, it would be, more than likely, very time consuming to do it right in a machine.