Do the QDC idea it works great as I have used it a few times. I use a G1/4 threaded Koolance VL3N on a T-Block and then have a barbed VLN3 in a piece of old tube. When I need to drain I put the hose in a jar, 1/4 twist and it is hooked up. Then I open the fill port on my res to prevent a vacuum in the loop and out comes almost all the water. My 360 rad is horizontal so I have to tip the case a bit to get the last bit of water to get out of that rad.
Koolance also makes an in-line drain port that has either compression fittings on either end or G1/4 for your own fittings. But it has a little screw off port for when you are ready to drain the system. I personally don't like that solution as much because you have to make sure you have enough tube on that link to pull the tube out of your case a bit in order to get the drain over a bucket.
I love the QDC way of doing it because I don't have all that extra tube just sitting in my case and don't have to worry about unscrewing a stop plug over a bucket and hope water doesn't go everywhere while I am unscrewing.