I just realised what the 'fake zoneline' in S Ro is...

[Jimjam]

After many years I have finally realised what the 'fake zoneline' in South Ro is meant to be...

It's a dry river bed running through the mountains!

This theory is confirmed by looking at art maps of Norrath; on the maps you can see a river cuts across the Oasis of Marr, and it is the bed of this river (dried out in game) that you can see producing the appearance of a 'fake' zoneline.