I've implemented this for Categories and Full Page Category Maps.
For now, I am considering this feature BETA and we can make adjustments as necessary over the course of time to make it fully supported.
On Category Pages for the small map, there is a "View legend" link in the footer of the map (as shown below). Clicking on this launches an overlay that will list all markers for the Category and all of the categories descendants (the entire category tree). The legend will display either custom markers or the default markers if the category does not have any custom markers set. I am using the Category Content Term as the TITLE of the Marker and if there is no Category Content Term set, it will use the Category Title. I am also displaying the Specific Category that the Marker is associated with.
Note: The screen shots are of RMS, however, its the same exact implementation in Showcase
Note: This is using Core XF Responsive Data List HTML/LESS/CSS Classes, so Narrow Responsive View has a slightly different appearance (and the Category Title is hidden in Narrow Responsive.