You were doing it the correct way, the only problem is that you were doing findClientByName(%client).
When you have a serverCmd, the first argument is always a client object, so there's no need to use that function.
Instead, you would just do %client.player.getPosition().
To set position, typically it's good practice to use %player.setTransform(%pos SPC %rot);, but just setting %player.position works.