I think a skills challenge is probably the way to go. They needs something that takes out the tackling and 1 on 1 physical component. I can't think of a way to make the players give enough effort to make it enjoyable to watch. No one wants to get hurt in an exhibition game. Flag football still has a level of risk to it. At least with the skill challenges you get to see some of the personality and I used to enjoy watching those challenges and things like the obstacle course runs. The players seemed to enjoy it as well.
What group of players could you possibly motivate to play hard? Free agents.. they don't want to get hurt. Rookies? maybe, but teams might not like it. I just don't see any way it can work.