Outward worship expresses interior faith. Abraham and Moses believed, "the Messiah WILL come", but we believe, "the Messiah HAS come". Thus, the rituals of the Old Law point to the Messiah who had not yet come, but the ceremonies of the New Law point to Jesus who has come and who will come again. Because the Messiah is come, it was necessary that the ceremonies of the Old pass away as they are fulfilled by the New.
Thus, it would be a form of false worship for a Catholic to participate in the ceremonies of the Old Law -- because this would imply that the Messiah has not yet come, and that Jesus is not the Messiah.
From this, it clearly follows that the Catholic cannot participate in the Jewish Seder meal at Passover time. Further, it is also inappropriate for Catholics to perform a "Christian version" of the Seder meal (the US Bishops explicitly forbade this in "God's Mercy Endures Forever" in 1988).
It is also good to realize that the Last Supper was not a Seder meal, as the Seder didn't even exist at the time of Jesus. What was the Last Supper then? It was the Mass!