Yep, that's how I would do it, just photograph them with a macro lens using a light box. Make a little jig to hold the slide above the illuminated surface so that imperfections and dirt on the light box are out of focus and don't become part of your copy. I've done a couple thousand copies this way.
You will usually get better results with a dedicated slide scanner, but not always, and the difference is usually hard to see. And scanning takes much longer. Plus, you need a scanner. I already have the scanner (Nikon Coolpix) but still go the copy route because it is much faster. I redo selected slides in the scanner as needed.