I have made a sketch of the visualization functionality, which seems to work fine. Unfortunately, there is a problem with modal dialogs (see here) so I had to turn this tool into a non-modal dialog; the code then needs more modifications in order to be crash-proof, so I will not publish it officially for now.
please check if it does fulfil your needs.
note: I have decided to use fake selection blocks in order to visualize the slices instead of loop markers. I think this solution is far better and, although the initial selection gets restored if no slicing is done when the dialog closes, it’s anyway more error-proof