I followed the instructions for the replace method, adding the line of code right under where advised:
<!-- Created with the Gal Civ 3 Editor -->
<!-- StarSystemDefs.xml -->
<!-- System Group -->
<StarSystemGroup>
<InternalName>FactionStart</InternalName>
<StarSystem>
<InternalName>ValSystem</InternalName>
<DisplayName>ValSystem_Name</DisplayName>
<Description>ValSystem_Dec</Description>
<OrbitLane>
<LaneType>Star</LaneType>
<Body>
<BodyType>UnaryStar</BodyType>
<BodyDef>ValStar</BodyDef>
</Body>
</OrbitLane>
<OrbitLane>
<LaneType>DeadZone</LaneType>
<MinPlanets>0</MinPlanets>
<MaxPlanets>1</MaxPlanets>
</OrbitLane>
<OrbitLane>
<LaneType>HabitableZone</LaneType>
<Body>
<BodyType>Planet</BodyType>
<BodyDef>PlanetVal'Om</BodyDef>
<IsHomeworld>true</IsHomeworld>
<Position>5</Position>
</Body>
<Body>
<BodyType>Planet</BodyType>
<BodyDef>PlanetVal'Dest</BodyDef>
</Body>
<Body>
<BodyType>Planet</BodyType>
<BodyDef>PlanetVal'Qain</BodyDef>
</Body>
</OrbitLane>
<OrbitLane>
<LaneType>NoRandom</LaneType>
<Body>
<BodyType>Asteroid</BodyType>
<BodyDef>NormalAsteroid</BodyDef>
<Position>0</Position>
</Body>
<Body>
<BodyType>Planet</BodyType>
<BodyDef>PlanetVal'Sur</BodyDef>
<Position>16</Position>
</Body>
</OrbitLane>
</StarSystem>
The vanilla systems then follow this. I've created files for each planet and linked them to existing art files, but I never see it in the drop down.