Given the project goals, I guess most encoders don't make a lot of sense: For image encoding you basically provide an "x * y * bytes_per_pixel" memory area and an encoder does its magic on that. There isn't really any complicated untrusted input in that case.
However it isn’t obvious whether such companies have any incentive to share their code, even just because the code is too specific to their hardware?
Certainly in number of images (as opposed to number of bytes), those might tilt things the other way.
There isn't one yet, but I'd like to have one at some point.
If you're encoding PNGs from NumPY arrays you don't care at all about safety.
If, for some reason I can't quite comprehend, whatever encoder you have on hand isn't fast enough, you can just disable compression optimizations -- but at that point you may as well be writing out a bitmap.