Why not use the TOC for that purpose? You can check the option 'visited slide only'. If you make it in Overlay, you could replace the expand/collapse icons by a transparent image. At the end of the course you make a toggle button visible that can open/close the TOC. Put that button on the first slide, and time it for the rest of the project. You can use the shared action described in this article for that button:
1 action = 5 Toggle Buttons - Captivate blog
The alternative would be to hide the playbar, and make it only visile at the end of the course. But that will mean that in the first sequental visit of the course they'll see an 'ugly' thick border at the bottom.