A workaround is that if you have an image, or non-transparent base layer, you can make a 'legend' set of layers below this, with no-transparency. You keep all of these on and because they are below the base layer, they will not display on-layout. They can then be added to and display without transparency in the legend. Not an ideal, but at least the legend then maintains its link to the main map without having to convert to graphics etc, which I have never found overly useful.
Hope this helps.
