I once encountered a name for this verbal practice that I quite like--'the obsequious concluding interrogatory'. I hope that a punctuation mark would not encourage this annoying usage pattern. I guess it would be useful for transcription. Or portraying a character in fiction. The other place I have encountered its use (besides the ValSpeak that you mention) is in certain sales representatives. In my experience sales reps that used this were usually based in California and worked in high tech. Perhaps others use it as well, as a technique to disarm objections in the prospect. As for the punctuation mark, I think your arrow notation is too strong and direct for what even you describe as a technique incorporating lack of confidence. How about a slash that rises to connect with the top left of a question mark? Like this: /? (except it would be one continuous character).