Maine Lobster Lunch in Portland
Not a big fan of Old Port Sea Grill aside from oysters and bar. If you're just doing basic steamed lobster I'd stick to the "basic" places, first for value, and second, especially for lunch, I've never thought the addition of white linen made a steamed lobster any better. I'd probably go with J's Oyster (ironically, I'd avoid the oysters but do lobster and steamers), maybe Porthole or Portland Lobster Company.
Agreed with bobbert on the OPSG--it's nothing special--and not really the right atmo for food that requires a plastic bib. If you're stuck on having your steamed lobster in Portland's Old Port, stick with the Porthole or Portland Lobster Co.
I saw you also posted about dinner/breakfast in Freeport in a separate thread: "Local, quality food preferred. Price doesn't matter." Portland has a lot to offer in this area. If you're amenable, you could switch up your meal planning and have a locavorish lunch in Portland somewhere like Eventide, Duckfat, Infiniti, etc., and have your steamed lobster for dinner in Freeport at Harraseeket Lunch and Lobster or Jamieson's Tavern.
Enjoy your visit!