Hey!
I don’t see a huge issue of exposing sockets that gives derivatives, even if the calculation might be somewhat expensive. We have technology which skips evaluation of unused sockets so simple cases will not be slower. Making complex cases possible is probably will be understandable that they require more computations. But surely would be good to hear from the rendering (and compositing) teams here.
The thing I am not fully sure: are you volunteering to take care of implementation, and just want some general answer from the rendering team yay-or-nay? We can give pointers in the code, but it might be a bit more trickier than “just” a shader. Not to scare you away, but more to set up expectations of the code complexity (it isn’t too bad, but might take a bit to get familiar).