Perhaps I just don't understand what you're proposing. If it's general enough to be presentable without any domain specifics, then it seems indistinguishable from a general algorithm like reversing a string. I don't use tasks quite as simple as string reversal but not far from it.

As I read it the main observable difference between your exercise and the kind of exercise you're arguing against is that you are starting with some baseline code that the candidate is supposed to modify. That seems like a pretty good idea, but I haven't used it yet.