Call it what you like or not, however unfortunately you're introducing a risk to yourself if the person you're passing suddenly decides to move to the left. The sort of person who sits in the middle lane is also the sort of person who is probably not paying attention to what's going on around them. It's especially risky if there's nowhere for you to move to should this happen. Not so much of an issue if there's at least a lane between you and the other car of course.
I agree that passing any vehicle introduces an element of risk, but I’d say the person driving irresponsibly in the middle lane on an empty road at a speed below the speed limit is the one introducing it. The only other options open to me in a situation like that are to slow down and not pass the lane hog, or to cross two lanes to overtake, and another two to get back.