Generally bikes with stems that don't have any forward reach benefit from bullbars or simular fitting. A Brompton response well to adding a dahon extender and low profile bars. The dahon extender is nearly 6 inches alone. If you can get the longer bars to hold in the handlebar clamp. Mezzo can't take this kind of extension due to their already massively forward reaching stem. The bike gets too twitchy. It work well with mini low profiles(see the mezzo thread), drops turned on their side and cut down. Cqg has fitted a alba santoria? Forgot it name. To his mezzo and this handles excellently. Dahon curves may have flex issues is you extend forwards,maybe mine was just a bad example.
Birdies have had forward reaching bars added by some posters. I own birdies and thought the handling worsen with the dahon stem so I remove it.
Any compact you are thinking of adding drops or similar to?