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The fill line is there to stop people doing what you plan on doing, but if everything is level load and won't spill, and a net can cover it, give or take a couple of feet you're probably within 'shrug' territory.
Issue isn't just height of load, it's weight of said load as well. On the other side it may be going onto a weigh bridge. Those extra feet are a significant additional weight especially if it's hard core. Can you keep it within a foot of the top of the skip? If the net can't cover it you may be stuffed.
It's less the safety aspect and more the cost for them to dispose that's the issue at hand.
What happens when you hire a skip, fill it way beyond the 'do not fill beyond this line' line by using two old doors as primitive dams holding back the diverse bricks, rubble, wood etc and the skip people turn up to carry it away?