I'm using rockwool right now, yeah. You could use hydroton in the netpots. I don't know about lava rock, etc. but I assume that any medium you see fitting in the netpots would work. I think the advantage of rockwool in the bloombox is the stability this provides the plant once it is fully rooted in the 3" cube. With big plants and hydroton, if you lifted the lid, I could see the plant shifting amongst the hydroton, which seems like it would be bad. Obviously any movement of the entire plant isn't good (stress) but it's somewhat necessary to get a look at the root systems later in flowering.
Are you still thinking about buying a box?
Are you still thinking about buying a box?