It already comes with a standard library, but, since the drawing algos left a lot to be desired, my watchface ships with its own image format and rendering code

It already comes with a standard library, but, since the drawing algos left a lot to be desired, my watchface ships with its own image format and rendering code to allow for handling of transparency and blending modes (since you can not only use the integrated render utilities, but simply draw directly to buffer with whatever you bring) So all in all: it has been done. That being said: you mention python, and I'm not sure where this is coming from. When in rome, you don't drag your favourite language throught the streets unless it is roman latin.