My idea is to directly link it to the difficulty slider. I strongly suspect that the people who do want DP are the same ones who want the game more difficult. The DP in itself makes the game more difficult by disallowing suicide as a tactic and requiring players to be more careful. Also ensures that the best rewards go to the players who deserve it and not the ones who just ramming speed every enemy in their path. Leave it off for the "easy" difficulty so that the people who do like cruising through the content can do it they way they have. Kind of like easy on The Beatles Rock Band, no fail is turned on when you pick easy.
Edit: Ninja'd. Took too long working on the wording.
Both you guys are geniuses. This would solve the problem, I'm guessing, almost entirely.