There's no function for when the brick's are done being ghosted, but there is one for when the save is done being loaded.
This doesn't really help at all. Go load a large save, and see how much longer it takes to ghost than it does to load.
Not exactly a callback, but you can check whether every object that is supposed to be loaded has a ghost ID assigned for said client.
client.getGhostID(%brick.getID()) != -1
I see, I didn't know about this function. I thought ghosting was purely client side, I didn't think the server would be able to get any info about it