One existing mechanism to attract new answers is opening a bounty. I'm not a big fan of the system, but just adding this to the discussion because I think it's

One existing mechanism to attract new answers is opening a bounty. I'm not a big fan of the system, but just adding this to the discussion because I think it's the closest thing stackexchange has to an official way to re-open a question for answering. One of the reasons you can select for the bounty is something along the lines of "current answers are outdated".