It's easy enough to post-process the image... Give the model a simple pure-white background. Export the PNG... In Photoshop / Gimp [or similar image-editor] choose the magic-wand tool and click any white pixel in the model's image to select all similar pixels. Hit delete and they are all gone. Repeat this with the background as necessary - e.g. if that's considered different color pixel-wise... Save changes - you should now have a transparent PNG with black line-work...