If I was to do that, I'd use 2 screens, probably both 280; one with everything on it, and the second with just the darkest parts. Your grays will come out nicer, and your darkest areas get 2 hits (one from each screen) and you end up with an extended tonal range that one screen generally won't give you. It's an old trick I read about in an old Kodak book for printers... some call him "Duotone"...
Steve