Matt Lewis, a member of Conservative Future, boasted that his "bad taste party outfit" included a blonde wig, pink pyjamas, a teddy bear and a vial of fake blood.
Other figures from the same organisation joined in the tasteless joke about the missing girl in exchanges on a social networking website.
One asked Mr Lewis: "Is this a cunning Baldrick style plan to obtain the reward money?"
Another friend of Mr Lewis said he planned to dress up as Robert Murat, the man who was wrongly accused of complicity in Madeleine's disappearance.
And others suggested going as the wife and daughter of Josef Fritzl, who were kept imprisoned in a cellar in Amstetten, Austria.
On New Year's Day, after the party, Mr Lewis sent a message to another Conservative Future member which said: "There was a brief moment when I thought I might have gone too far with elements of the costume but it was OK - someone else was Baby P."