I think you probably could do that, if you take a cab from the airport. There's a bus-bus-subway connection to downtown, but that's a pain and can take around an hour. I'm not sure how much a cab to/from the airport would cost, but it's not likely to be terribly cheap - Dorval is a fair hike from the city center.

If you're in Old Montreal you could also get lunch in Chinatown if you're so inclined. The neighborhoods are very close together, and although small Chinatown has a bunch of tasty places to eat. There's a place called Hong Kong that I quite like - massive amounts of food and reasonable prices.