Dont worry, just get a blow-off tube for when the fermentation kicks up foam on top of the beer. You can expect roughly 2-3 inches of foam at its most active, though I have seen belgian yeats cause an excessive amount of foam that went straight through an airlock. Basicaly, just hook up a 18-26 inch rubber hose to the end of the plastic tube in your airlock, and put the other end in a mason jar half-way full of vinegar, water, or whatever sanitizing solution you have available.
I would just fill up the fermentor to where the curve begins, and ditch the rest of the beer. Cheap solution.
If you fill it to the neck, the pressure could pop open your fermentor, and although open fermentation is possible, and CO2 can act as a protecting layer on top of a fermenting beer, your still at risk for infection.
Fermentation is pretty powerful when really active. I have a stainless steel 80L fermentor, and when I ferment using a good, healthy culture, it can shake from the bubbling... to the point of keeping me up at night.