I don't know how often you plan on having these or if this is a one time thing, but something like four times a year would be pretty cool. Then if you were to just solicit pics that were taken during the three months since the last contest, that would keep everything current and fresh.
I have come to know an awful lot of Flickr members who take event pics, and judging by the amount of pride they take in them, most would probably jump at the chance to enter them into a contest. If I get the word out, this could be the catalyst bringing lots of new members and pics to the forums (of course the pics themselves end up here anyway, but at least they would be posted by the original photographers now
).
Klondike