I did this just tonight. Even without stopper, it works great. A broken racking cane (curve broke off) fits into the picnic tap. Turn the gas off, and purge the keg. Turn the psi way down, to about 2 psi. Place the end of the racking cane into the tap, and the other end at the bottom of the growler, and turn on the gas. Fill very slowly, as to reduce foaming. It seems counterintuitive, but keep the tap open all of the way. If you "pinch" the tap, you'll get foaming. Keeping the tap all the way open but filling slowly reduces foaming.
For filling bottles, you can use the "we don't need no stinking beer gun" procedure, and you can even bottle for competitions this way.
One of the things I love about kegging is that I can still bottle, and when I give gifts of homebrew, there is no sediment at all to get stirred up. I carry growlers every where I go, and people love the "professional" look.
Broken Leg Brewery
Giving beer a leg to stand on since 2006