I tried this a few months ago. I cut out some ~ 1/8" aluminum sheet on the shear at work and placed it on my prints and flashed after exposing the print. I was unhappy with the results because the edge, despite being straight was not as crisp as i had wanted. Perhaps if I flashed longer or had thinner sheets it wouldhave been better. I even tried using a LED flashlight to burn in the black border.
I was hoping to get a black border between the white border from my easel and the exposed image. I eventually gave up though i see no reason that I couldn't have gotten it to work.