I've put together some code that generates a badge for coverage %.
At this stage, it uses a pre-generated batch of images, but mainly because that was faster for me than to learn how to use ImageMagick or PIL to add the text to template images.
The big problem is that there then need to be 101 images for each style of badge. The badges I've used match drone.io's passing/failure badge (kind-of).
Solutions to this would be a template image that has text added, or ranges rather than % figures. Obviously, it would be best to be able to generate badges based on a generic template image, which could be supplied.
Is this something you would consider including?
I have it in use on: https://bitbucket.org/schinckel/django-multi-schema/overview