It occurred to me, I could make alternative versions of the trader POIs that had no trader, but zombies instead. I'd append an "x" onto the name of the POI and rig them not to get placed by RWG. Then you could edit a map RWG created and append the "x" onto the POI name and you'd swap a working trader for a dead trader that was overrun with zombies.
For instance, you edit your world's prefab.xml file, find this:
<decoration type="model" name="trader_bob" position="1605,34,-1306" rotation="1"/>
And change it to this:
<decoration type="model" name="trader_bobx" position="1605,34,-1306" rotation="1"/>
And you'd be done, so long as you had the alternative versions of those trader POIs.
I'll try it, and if it pans out, I could (likely) get the alternative traders into the next release of my modlet.
UDPATE: The concept works. I just need to work on the POIs.