So this was a question I saw on Reddit, but I thought it should get some treatment here. What do we do at Regular REL when a player forgets their scry after a mulligan? My gut reaction is to let them have it if they have not drawn their first card of the game.

As its regular - go with your gut. Let them scry if they haven't drawn. I'd even consider some circumstances where I'd let them scry after they'd drawn (if they were on the draw). It's going to take a while for people to get into the new habit. Your best bet is going to be plenty of announcements before and during the event, and encourage people to remind their opponents.

The scry is an optional action. Even at Regular, we generally assume missed optional actions are missed and don't let the player go back to do them. That said, if the player hasn't drawn their card yet, you'd likely have a hard time convincing me that the genuinely missed it. The opponent shouldn't be able to rush them out of a scry by playing their first land quickly.