With just XML you cannot accomplish it too easily. You would have to make a second unit in the unitclass of each hero, and have it be bGraphicalOnly, probably named "Hero" for the Description, set the dummy hero as the default unit for the unitclass. Then in the CivilizationInfos you would list the proper hero as a replacement for the dummy-hero, and it SHOULD display in the pedia at that point "Replaced Hero for Amurites" or whoever you are dealing with.
Cludgy, but it would be a pure XML approach. You could use python to directly list each hero and force the display, but it would be a tad tricky to make it appear like it was automatically placed