Yo!
This is an actual service, that happens to be in costume, yes. And hey, having a Seder sounds awesome, I'm glad other people are doing stuff like this (my brother is Jewish by heritage, not religion, but we celebrate Passover too, so we think this is really cool!) - And also, this is an unofficial thing, fan-started and conducted, so you are right, it is at the Con, by and for people at the Con, but it is not an official Con-sanctioned event, which is fine with us. There might be potential issues with the Con actually sanctioning a specific religious event, so it's understandable. And yeah, we are people that believe in and follow the Christian God, but we also love anime and manga and find great truths and insight from Eastern tradition and philosophy as well! Thus far, it has been a really nice thing doing this service, and considering how much anime and manga really explores religion and philosophy (including Western traditions, as in Fullmetal Alchemist, for instance), it does actually go very well together to hold a service and also have some exploration of different beliefs (both Eastern and Western). I hope this clarifies things a bit for you guys, thanks for posting, and of course you're all invited to the service if you're interested or curious.
I look forward to meeting you all at this Con, which sounds like a blast!
Yours,
Daniyel