I had a little look into this. It looks like OnClientConnectCallback happens too late to be useful, you may be able to use an in-scene object on the login screen as it looks to spawn first, unless the player object is auto-spawned as this appears to happen before other spawns.
A better solution may be using the scene manager’s callbacks. Sending a custom message in OnSynchronize arrives on the client before the scene objects are spawned so it sounds like it might achieve what you’re after.