I think it is a good idea but you will not have any success with it.
Most of the campaigns now only require a certain number of posts because they are after the awareness.
Most of the participants do their posts (this is the main problem of spamming) without any research in order to get to their post count as fast as they can.
So if I have to choose from a campaign that requires post count and your campaign for sure I will go towards the first one because is simpler to achieve.

Dont get me wrong, you have my full support for this but in an ecosystem based on spam for the bounty I don't think it will work.