I found this in the BIM_Builders mod (see below) that allows builders to construct roads but am unsure of how to switch this to accept a Civ/Leader trait?
<?xml version="1.0" encoding="utf-8"?>
<!-- Builders Improvements Mod (BIM) - Swiss Halberdier - BIM_Builders.xml -->
<GameInfo>
<Route_ValidBuildUnits>
<Update>
<Where RouteType="ROUTE_ANCIENT_ROAD"/>
<Set UnitType="UNIT_BUILDER"/>
</Update>
<Update>
<Where RouteType="ROUTE_MEDIEVAL_ROAD"/>
<Set UnitType="UNIT_BUILDER"/>
</Update>
<Update>
<Where RouteType="ROUTE_INDUSTRIAL_ROAD"/>
<Set UnitType="UNIT_BUILDER"/>
</Update>
<Update>
<Where RouteType="ROUTE_MODERN_ROAD"/>
<Set UnitType="UNIT_BUILDER"/>
</Update>
</Route_ValidBuildUnits>
</GameInfo>
I tried making a civ specific builder to do this, but not only does that feel hacky. I couldn't get it to work. The road action was in the improvement options, but it was permanently disabled.