Actually, the mechanism is postulated but not yet proven. Avastin is a potent inhibitor of angiogenesis, or the formation of new small blood vessels. It works on cancer because it starves the cancer of its blood supply. Part of the mechanism of retinal damage is the body's attempt to grow new retinal vessels, or neovascularization. Avastin inhibits this, thereby minimizing further retinal damage.
You are correct, it is not yet FDA approved for this but is undergoing trials.
That, and the fact I had a closed head injury back in '99 kinda ruined my FAA medical. I miss flying, but for some reason, they take a dim view of those with frontal lobe damage sitting in the pointy end of things which go very fast LOL..
Greg