Couldn't you technically already do this? There is a semi-glitch in the JVS system where if you save with a JVS switch in the 'on' position, then save, the next time you load the save, that switch will switch to the 'on' position again. Therefore, all you need to do is create a switch that does everything, then leave it in the 'on' position.
But ya, onplant would be helpful.
Infact, a whole line of brick-condition events would be helpful, such as Lugnut's 'load save event' that loads the specified save. I'd like to see 'save build event' that saves the entire build upon input, 'copy events' which copies the specified range of lines of events to the specified brick or the brick in the specified direction, and 'delete brick' which deletes the specified brick or the brick in the specified direction.