Can't render previews in headless mode

There's no documented command to render a preview in headless mode.
I expected that setting "pathTrace" to false in the json would cause renders to be preview quality, but that's not the case in my tests. Which makes the "pathTrace" option seem a bit pointless.

I do have a use for those flat previews. I will be rendering two versions of every scene - one is a high resolution preview, the other a low resolution raytrace - and combining these into a final image that looks pretty good and renders much faster.

I should mention that I'm using 1.3.8 on Windows 10.