Wouldn't it be possible to do this already with an InSim app on the server? Give a speed limit to the leader and if he/she goes faster than that then give a penalty (Display a race control message to let the driver know about it, "Keep your speed under 100km/h" or something.) and when the leader passes a certain point in the race show a "green flag" message to everyone. If at this point someone has passed another car then give him/her a penalty. For the non-leader cars there could be a "Stay behind driver X" message constantly displayed before the flag goes green.
Obviously it would need some other checks too, ie. if someone stops during the lap then throw him to spectating so others can pass him/her.