This question came up in the irc channel yesterday. The fellow had a model he was turning into a sprite for a 2D game. He wanted it to be aliased inside, but have a hard (aliased) edge.
Getting transparent images is easy. Go to the Render Properties panel and set your Alpha to Transparent and make sure the file format is saving RGBA channels.
Getting a hard (aliased) edge is easy. Turn off Anti-Aliasing in the Render Properties. This gives your model hard edges, even in the internal pixels.
Now if turn on anti-aliasing you get soft edges, both inside, and outside.
How do you get hard edges on the outside, but soft edges inside?