Nah, if it's done this way people will lure the boss outside the boss room, just to troll players. Which means they could find some place where they can hit the boss, and the boss can't hit back.
The most sensible thing to do is to not spawn the boss in the boss room until the entire party is in the boss room, or the ready check has a 100% inside the boss room. Failure to do either, the first player who crosses the boss room door line, begins a 10 second countdown where everyone is teleported into the room and locks it immediately, cutscene watching or not. I can think of at least one duty (Castrum Meridianum) where that would solve problems where players get locked out because of players pulling the boss while adds from the previous part of the dungeon are still going. The reason that is unworkable is because the Alliance raids often have 3-way splits, so you don't know where the party should be split up, but the game also shows it's hand about how to handle this because it does teleport teams around in Ozma, and a few other Alliance raids have mechanics that target specific teams to separate them.
The issue with the Alliance raids cutscenes has more to do with how many players are actually needed for "old" content. When the content is new, you want as many players as possible to burn down the boss fast. If the content is old, an entire team can be left outside the boss room and it will still be completed.
Edit: come to think of it, the entire reason it works the way it does now is to prevent people from doing a wall-to-wall monster train to the boss room, and having the monsters get locked out to avoid having to defeat them. Maybe instead don't spawn the boss while cutscenes or party members have enmity upon crossing the boss room door, and if it's just a cutscene, teleport them inside the boss room.