Purim torah in context can be not only fun but educational, as the best purim torah uses the same methods as discussions on more-serious topics.
When purim torah comes up on mailing lists, everybody knows what's going on because of the timing and there's no confusion. The challenge with purim torah on SE (or a blog for that matter) is that somebody might come across it six months later via Google and not notice the timing. A purim-torah tag helps, but may not be enough for people not accustomed to looking at tags. SE users grok tags; random Googlers might not notice.
So my suggestion is: permit purim torah for some span of time (not just the day of Purim, when many people will be busy), require the tag, and after the period is over, editadd a simple (one-line) disclaimer into the top of all the questions for the benefit of future browsers.
As for the timespan, all of Adar might be too much but just the day of Purim is too little. I suggest One to two weeks seems good to me. (If a week -- and perhaps this, we could bemake it the weekly topic for that week.) In the comments @msh210 suggests Adar (bet) 1-17, which seems fine to me.