I don't see why not, it worked for Nemsy so it should work for this. Also, you can open up your packs too if you want, so yippee for that I guess? Honestly though I feel like this idea of a pre release is just stupid, gets rid of all the fun you can do in terms of an actual release (going by MTG prerelease standards, which are normally all pretty fun).
Edit: now that I read the thing, turns out you can only use cards you opened up from packs for this, can't make or use any cards you didn't get, seems a bit scammy imo.
Hey, can you get Nemsy with this trick? Just try with any account and give some feedback. I can't do it anymore. I can join any Fireside, but I can't start a match. Some error occured, and the match was closed immediately. Look like they add some verification steps, and prevent this trick.