I was wondering if it is possible to create an outline only render on a transparent background.
I can start with the Workbench renderer, using flat shading, single color and outlines enabled, which comes close, but I would like to get rid of the colored fills as well. Unfortunately I cannot disable the color completely. Is this possible?
Try this: Set the material and background colors to black and set the outline color to white. Then go to the Compositing view and map the brightness to the alpha channel.
Indeed, the node-based Compositor will be where you can achieve the final results that you want. Here, you can indeed eliminate the colors – or, completely replace them.
In this case, you can simply use the Alpha channel, which represents transparency. Everywhere the girl is … is “opaque” (Alpha=1.0). Everywhere the girl isn’t … is “transparent.” (Alpha=0.0) So, bingo … “there’s your outline.” You could simply grab that output, mix it with a color-node that directly produces the shade that you want, and use that as your output … ignoring completely the one produced by the render! (But, do pay attention to “anti-aliasing” along the way, to avoid “jaggies.” Batteries are included to handle this.)
Yes, the compositor is “the digitally perfect darkroom.” In which you are free to creatively concoct any sequence of processes that you can “plug up.” 
Make it your business to thoroughly learn about this tool and all that it can do for you.
Thanks guys, even though I was hoping for some option on the renderer, this works fine for my purposes.
