I really like this idea. And it could easily be done with just a category topic or hashtag for truthordare. Say for example I posted a dare for someone to link to a video of themselves ridding in a shopping cart or something silly like that, then the first person to link to such a video in the reply section would win whatever amount of Steem that I pledged in the dare post..
There's no need to make any additional platforms to automate anything, it could all be done here on SteemIt. If someone doesn't pay what they pledged for a dare then that person might get downvoted.
There might need to be some rules to verify that the person in the video is the same person posting the link, but something simple like holding a piece of paper with the word 'Steem' written on it next to the current date or something would probably be enough for starters.

And I would totally be a player :)