Haven't but you reminded me I have some jumpers I was going to alter. They had polo necks which i find a bit too hot, I bought some Liberty lawn and was going to replace the neck with a collar and placket to make it easy to put on. Must have a go at that. Thanks for the reminder!
I do have a tee shirt which has a mock shirt sewn inside the V neck. The binding is left loose around the V so they look separate. I think I would do the same on your top, sew along the inside edge of the ribbing or whatever finish there is, to leave it looking loose. Oh, how difficult it is to describe the picture in ones head.