I had a US mil pilot request F420 in oceanic airspace once and thought the best action was to clear him into a block F410-F430 as those two levels are unavailable to other a/c. There is also a good chance the a/c is operating close to the limits of its flight envelope so a little room for manoeuver also helps. Easy to do over sparsely used ocean but not really a goer in high density airspace, where I wouldn't accommodate such a request.