No manager that takes this game even remotely seriously is going to throw a match to win a 50k prize on the off-chance (or even gauranteed chance) that the person he picked to win the league actually wins. And you say that in an inactive league it is an advantage, not necessarily so, as there may be less active teams but there will be just as much competition between these teams. Also, 50k is not a big advantage, it is simply a novelty prize.
Getting teams to pick a winner prior to the end of the season for no reward is like playing poker with play chips. There needs to be some incentive to pick the winner, even if it is something as small as 50k.
As for it being hard to implement, I don't know if it is hard or easy to implement. Obviously I would rather have things such as a new draft system implemented before this, but that doesn't mean this would not be a good, or at least decent, addition to the BB site.