AFAIK there's no forum rules that prohibits this kind of reward, but this will somewhat force sigcamp participants to make more post which might end up to spam.
There will be no problem with this if the manager chooses quality posters,
[there are tons of ways to raise the bar for participants]. In addition, after a couple of weeks
[or before acceptance], the campaign manager will easily be able to
check how the activity of a particular user has changed
(to know if the choice was successful and who really deserves additional rewards).