How to get crisp on/off alpha AND anti-aliasing?

I’m making graphics for a very old isometric game, which uses colorkey for transparency.
Having smooth edges near the background color make the edges look really ugly ingame.
Disabling oversampling gives me a nice 1-bit transparency but reduces image quality significantly.
How can I threshold alpha to either 0.00 or 1.00 and still have anti-aliasing?

I finally succeeded after messing with composite nodes. Here is the node setup:

Ok, under the shading drop-down menu change alpha from sky/pre-multiply to straight alpha and then the Anti Aliasing will work on the image but there will be a set alpha value. Note, make sure you have AA set to 16 otherwise you wont get fully smooth edges on the side. And for the AA filter I recommend Catmull-Rom.