To date I haven't found a source for this legend.
Actually, it is not clear from all sources that Eliyahu actually visits the Seder but rather that it is only symbolic that we hope and pray for him to come. There are sources though who do indeed feel that Eliyahu visits the seder similar to a bris and that we anounce Boruch Habaah! at the time of the door opening to welcome his visit. No mention of sipping the wine though...
I would not dismiss this legend out of hand though. More research is required. My experience shows that minhagim held as true by a good portion of klal yisrael usually started from somewhere. We just have to keep looking.
Hope this helps,