In the end...
Well, I was looking at the calendar a couple of nights earlier, and realized that the 15th of March fell this year on the 15th of Adar, just when I happen to be living in Jerusalem where Purim is celebrated on the 15th rather than the 14th. And so, with a little assistance from Aviel in the research of ancient Roman garb, I entered the spirit of "v'nahafoch hu" [Literally, "and he/it was changed/switched/flipped; the concept that on Purim, both in the original story and hence at this time every year, things are turned on their heads. Irony and poetic justice reign.]
The funniest costume idea I've heard of recently, though, is from a commenter to the Renegade Rebbetzin: "One year I made a crown, and had a picture of a turkey in one hand (I couldn't find a rubber/toy turkey) with a Koosh ball in the other: ha-molech mei-Hodu v'ad Kush..."
Hahahahahahahahahahaha!!!!!!!!!!!! I wish I'd read that before Purim!