I went to a "Messianic Seder" years ago as a means to learn exactly how the Seder pictured the work of Christ, and it was absolutely striking in it's detail; from the breaking of the middle of three loaves (if one lets the loaves represent the members of the Trinity, the middle one being the Son, broken for us) to the way unleavened bread forms stripes when it is pierced and baked.

As an educational demonstration of how the Seder represented the Lamb of God's work on the cross, it's wonderful! Observed as a sacrament, however, I would definitely have issues. The first Lord's Supper was a Seder meal, but as a sacramental means of grace, it must be observed in remembrance of Christ; whereas the Seder looked forward with faith to Christ.

-Robin