Google's "image search" has been completely destroyed by Pinterest anyway. In that vein, this doesn't make much of a difference.

I'm curious, in what way? I can not look at any Pinterest content because I don't have an account with them. Figured most people would relent and sign up -- but perhaps this is the problem you were talking about?

Pintrest spams Image Search results with bogus results and uses them as the top of their funnel.

You click a Pintrest image unsuspectingly, Pintrest takes you to a vaguely relevant board the image is contained on, but you can't get to the image because a signup lightbox blocks the content.

This has wider implications because using reverse image search wont point you to the original images either anymore it'll mostly point to Pintrest results and therefore Pintrests signup form.

Have you tried using search operators to exclude results from Pintrest?