Is this a huge problem? I understand some people will try to exploit, but the few that tries should get a ban in return. Toggle it during contests if need be.
Alternativly, extend the edit time to a reasonable time, like a few hours, and limitless for the creativity forums as suggested above.
I only ask since I have not seen this being a problem on a domain with no limits and high traffic. Try it out and check logs of edits of old post as a test perhaps? More work for mods, but mods generally have little to do compared to a few years ago, so this will go down well