Worldgen mod question

[danielzklein]

Hi! I'm trying to mod worldgen such that marshes are less likely to spawn. I tried reading through the luas I thought my be relevant, and my first conclusion was that I'd set the likelihood of marshes spawning in:

data/scripts/map/rooms/terrain_marsh.lua

I tried adjusting the distributepercent for each room type defined in there (BGMarsh, Marsh, SpiderMarsh, SlightlyMermySwamp), but what that seemed to do was adjusting the amount of stuff spawning INSIDE the marsh (trees etc). 

Tracking through forest_map.lua my best guess is that the actual map creation happens in WorldSim:GenerateVoronoiMap, and that that happens in C++ (so invisible to me). 

Is there a way to adjust probabilities for individual biomes? Am I even looking in the right places?