SWP 6.0.24 on MS Win
It doesn't seem possible to add custom labels to the automatically generated equation labels. The Help file says to select the label, then right-click, select Properties and then Equation Array Properties.
When I select Properties all I get is a little square just above and to the right of the >. No submenu with Equation Array Properties.
This seems to be the same bug as reported quite some time ago in the context of array properties. It's a pity that it hasn't been fixed.