When Global and Side chat is disabled that leaves Group, Vehicle and Direct options working.
That means that for an in-game administration program like BEC or B3 to be be able to 'see' what an admin types they must either be in a group, in a vehicle or close to another player which is not always possible. This is because Battleye only seems to broadcast text chat via rcon if another player 'hears' it in game.
We would just need an 'Admin' channel that BE would broadcast over rcon, but would actually not go to any player, except maybe the admin themselves.