I'm completely biased here, but I think it would be a shame to entirely exclude maps that had been worked on before the contest announcement. It wasn't clear when or even if there would be another contest and I'd rather not have to pick putting my current work on ice for a year or not taking part in the contest. I completely get that having additional weeks or months can seem very unfair, but I think the area where it'll show up most will be polish. 9 months should be enough for any team to make a solid layout and some pretty good art. A level that was started prior to the contest would be expected to be much more polished by the end of it and that could be reflected in the judging. For example, if 20 points are given out for polish and overall quality, then it could be set to max. 10 points for maps that were already being worked on.
I wasn't participating in the last contest, so I'd be curious to know how much the "already worked on maps" rule really contributed to the finalists and winners. Did anyone really get their position in the contest by simply submitted a map that was already half done?