I'm not quite sure how I feel about this.
If someone was actually moderator for the day, I don't see it going very well. Either they have complete control over a section like a normal moderator and let's be honest, some people aren't cut out for that power. Or, if they still have control, they don't like how the current moderator is running things, so they decide to uproot all the original moderator's work and start down another path. There will be some people that will abuse the power and close threads they don't like, troll their friends, annoy their enemies, and just abuse the power that comes with being a mod.
On the other side of the spectrum, what if they don't become a mod, but work very closely with the current mod. Well, that would just be the same thing as a normal member and not really a prize. Maybe there's a middle ground, but I just don't see it happening.
The final issue I'm seeing is that this contest would make being a part of staff a "prize". Like it's some sort of shiny medal you should be striving for. I don't think that label for staff should exist.
Anyway, those are my first thoughts and are subject to complete change once other people voice opinions and perspectives that I haven't thought of.