fuel dumping is pretty well understood...
but:
most planes can't dump fuel. the 747 certainly can. A rough guide is really big planes can...smaller planes can't....some 767's can, some can't. A few very rare DC9'30's can...but most can't. Boeing published a detailed list of their planes, and McD's planes too.
When the first experiments of dumping fuel were done, the fuel was dumped over an open fire on the ground to see if the fuel would ignite...remember its vapor by then.
It didn't catch on fire....and the altitude of 4000' agl is generally accepted as minimum dumping altitude.
on the gap departure, one is over the pacific ocean quite soon after takeoff.