Well, seems you could do 2 things:
1) See if a gunsmith or maybe Teague would fit screw in chokes for you
2) Get an interchangeable choke barrel. I think Saddlery and Gun Room are Mossberg (Maverick) importers
One issue with the 870 is that the lifter / loading gate stays up as is spring loaded. This can be a hassle when trying to load I understand. Benelli's and Mossbergs have agate that stays out of the way.