teach git diff -v/--invert-match?

I'm wondering what it would take to teach git diff to invert the -S string (like git grep).

I'm finding git diff -S<string> [--pickaxe-regex] to be really useful, but find I have cases where I want to ignore differences. For example, I might not care if the only changes to a Java file, for example, are related to import statements. I'd like to be able to do something like "git diff -S'^import' --pickaxe-regex -v". I'll admit I can get by with something like "git diff -S'^[^i]' --pickaxe-regex", but am pining for -v/--invert-match.