I played LAN with my brother, I created a server via my pc. during the game I run Tweak and I change some option, and after my brother and disconnected from the server and can more input.
Yes, it is allowed. You can do same with creating start (1m space) 1st checkpoint (1m space) finish-type layout and doing some "laps" on it.
Your brother has to have SAME options. e.g. if you changed your XRG's power to 1.7, you must change it on your brothers PC to 1.7 too.