I've not tested it, but i've made a similar plugin (add storage boxes to scrap heli), and after server restarts `OnEntitySpawned` is called for each copter. at the time its called it might not have the `CopterSeating` component loaded, so either wait/check for `OnServerInitialized` or defer the checking for `mountPoints.Length` and seat adding with a timer, so you don't spawn double seats over another :)

will look into it soon or you can always send in a request on github