A room variable has an optional 'locked' property. If set to true then that room variable cannot be overwritten, but I can be deleted.
Based on the rules that you mentioned above, you should be able to have everyone try to set the room variable when they enter the room with locked=true. The first one received by ElectroServer will work. All clients will receive the appropriate variable name/value as a response.
This is directly from the documentation:
var ob:Object = new Object();
ob.name = "secretDoorUnlocked";
ob.data = "true";//this is a string because all room variables are strings
ob.locked = true;
ob.persistent = true;