Have you experimented with manipulating the image before trying to chroma key it?
For example, converting it to HLS or HVS spaces with a ColorSpace node. Even if there's no difference in RGB colour or brightness, there could be a difference in saturation or some other area that might not be immediately visible to the naked eye. It's worth a look at least.
VFXStudy did a good video on it which will show some possible techniques:
I'm not saying it will work, but it's something to test out before resorting to manual roto. It's possible it might at least get some of the way there; eg it might give you a key that selects 50% of the object, or selects 100% but with some holes, and then you need to roto the remainder. Which could prove to be less work than rotoing the whole thing.