If you want to make a contest, please get "community approval" first.
I read in MSE that community can decide the policy of what's acceptable themselves (some sort of individual states' law in US?), so if an event is approved, go ahead.
How to get community approval?
Simply post a meta, with the details of your event/contest. Of course, if you want approval by the community, provide reasons why you think this is beneficial for the community, instead of just throwing random bounties.
Be prepared to take criticism.
For example, in [Interpersonal.SE], we have a 50-day and 100-days celebration of the IPS site.
In [Writers.SE], we have a Writing Challenge. The user explicitly ask community approval first before going with the Challenge.
Although community can bend SE rules if they agree, if you want your proposal to go through, be cautious if you need to do so. Provide justification why you need to bend the rules (for example: posting a question that usually will be closed as too broad, or too opinion based).
If you decide to use main site, usual rules apply, because we don't want to confuse users about this site scope and rule (except if community agrees to give exception to contest posts).
Thanks for caring about Worldbuilding.SE, and I hope your contest can go well!