They need to be registered ahead of a roster lock, unless there's something in the rules about making emergency changes, which there might be. At DH Masters (not an RMR tourney), Dima was allowed to stand-in for mir in Spirit when mir's electricity went out. Not all tournaments have the same rules and I'm not sure if RMR tournaments allow it.
Also, the Dima subbing was mid-game, which might be what qualifies it as an emergency substitution. This would be a substitution pre-game, which might not count as an emergency.
If stand-ins aren't allowed, B1ad3 will be forced to play instead of him. If they are allowed, we're probably gonna see B1T again.