Why can't people use any dataset they want? I think the rule should be, "Use any data you like, but you must submit any data used for use by the rest of the field".
If you have other suggestions, please let us know, and we'll add it for next year (if possible). The only thing we're trying to avoid is somebody coming in with a lot of data at the last minute, beyond the point when anybody else can realistically get it incorporated into their model.