Sorry no one has chimed in with suggestions. I don't live downtown, so I don't have much experience with take-out there.
The first idea that springs to mind is sushi, as nearly all sushi places do take-out. My money usually goes to Tokiwa on Bishop, 2/3 of a block south of Ste-Catherine. Hot and Spicy (I think that's what it's called), a Chinese take-out on the ground floor of the Faubourg Ste-Catherine, is popular with locals though it's hardly gourmet. Would be surprised if some of the Indian restaurants didn't offer take-out.
One last tip: there's a delivery service--and I'm sorry but I can neither recall the name (they don't deliver to my neighbourhood) nor find it in the Yellow Pages--that works with a number of downtown restaurants, some of them quite fancy. Prices are high. Maybe someone reading this can point us in the right direction.