Took a moment to play around with it. You'll probably not want to render over a totally white empty background because of the annoying fringing you'll get when you composite your work. For this I turned off the ground, loaded a very scaled up plane to act as my background, set that to totally black diffuse with no specular. You can render with the background set as black in the scene but this causes the sketch renderer to effect the foreground object, the focus, in a particular way. As you probably know, the 'Render Over Black' in the Sketch Designer itself also behaves not as typical but results in a negative render, sort of a scratch board effect. That's not what I'm looking for here. Overall I want to control and produce a black and white line layer and a color layer for maximum flexibility in compositing into an object on a transparent background. It's a few steps but each element takes literally seconds to produce so not a big deal.

First I turn off the ground visibility, load a backdrop plane, and make it flat black. For the sketch renderer I imagine you can just leave the ground on and make that all black. I personally create a lot of smooth shaded lined preview render images and a background object set to black (other than the ground plane/construct) is a matter of habit for me for various reasons relating to wireframe opacity and fill layers with preview renders.

I used a a woodcut setting for the line work in the sketch renderer. It's practically impossible to render out a completely solid black background (AFAIK) with the sketch renderer without seriously influencing the foreground object focus. I played around with the background settings as much as was feasible to produce the most dense effect...

For my final image I want a very strong black holding line to surround my focus. For this I turn to the comic preview options, Geometric Edge Lines, to produce the holding line. A magic wanded stroke layer in composite would suffice as well. I just produced the layer here in Poser. I turned off my background plane, changed the background to black, converted an infinite to diffuseIBL (you could simply flood the object with ambient also), and flooded the scene with diffuse. It's really just a few clicks. The holding line image...

Finally a quick preview image color layer... with a bit of shading

Over to the image editor I assemble my layers, do a couple magic wand selection cuts and blending modes (multiply/linear light)...

Merge visible (not Flatten) so I'm left with my object on a transparent background... save as png.