I would suggest a different approach. When there is a region, by definition point must be at the beginning or end of the region. Which means you can simple enter the text you want at the beginning (or end) of the region. C-x C-x will move point to the other end, where you can enter the second half of your wrapper.

You don't get explicitly asked prompted for the insertion text, but I can't imagine there would be a way to accomplish the same thing with less effort.

I would suggest a different approach. When there is a region, by definition point must be at the beginning (or end) of the region, and mark will be at the end (or beginning). Which means you can enter the text you want at point, and then use exchange-point-and-mark, i.e. C-x C-x, to jump to the other end of the region. Enter the second half of your wrapper and you're done.

You don't get explicitly asked prompted for the insertion text, but I can't imagine there would be a way to accomplish the same thing with less effort.