What do I have to change in the code to be able to mount the sign with /plate?

So everyone who has permission can decide for themselves whether they want to do it or not

Um. Not sure if you can. This attaches the plate to the mini before it is spawned. It might be possible to graft it on afterward, but an alternative solution is to use the permission system, and have a command to toggle the permission. So something like /toggleplate to toggle the permission on and off for that player.

Otherwise to add the plate to an existing mini, you'd have to have a raycast to hit the mini, identify it, add the plate to the mini, remove it's meshcollider etc, and then force a netupdate on the mini. I've not played with raycasting much, and never tried to add prefabs to existing vehicles. Not sure if you can post-attach stuff to vehicles. So far the custom stuff I have seen is all added before the item is spawned ingame. If you know of something that can add a child to a entity after the entity is spawned, let me know. If it's licence allows I'll see how it's done and can maybe alter the plugin to do that.