Try it. Honestly the game doesn't care, after reaching the star it doesn't care where you go to sink merchants, as long as you sink 10 thousand tons of enemy merchants they could be anywhere in the world. Ridiculous part is the parameters for keeping command (a few bad patrols will actually end a career) or getting promotions and medals are actually based on renown alone - and the most renown comes from tonnage.
Browse through some of the .mis and .cfg files, they're all readable in notepad, you'll see what I mean.
Stock game you get 200 points for completing the assigned mission, 1300 points for ignoring the mission and sinking a FUSO class battleship. They send you someplace where there are no targets, don't go there, you only lose 200 points from the total. Find and sink five NOL_Nippon large tankers, 1000 points each, so 5000 points versus 200? If they're gonna make the rules silly I'm gonna play silly.
I actually did try this once, played an entire career from 1941 to 1945 without ever getting credit for any mission - doesn't matter as long as the tons add up.