I support a general mechanism. But I think that if it's much more than the 2 characters that this would take, then I don't want to use it for f-strings. The whole point here (as with all f-string features) is a concise way to get the string you're after.

But if you have some ideas, I'm willing to entertain them. I want this feature to land in 3.8, for which we're rapidly running out of time.