I've attached my modded version. Cephalo, if you don't want this distributed, let me know and I'll take it down.
The original forest planting code works like this: the amount of rain on each tile is compared to a constant called TreeFactor (really it's compared to another constant multiplied by TreeFactor, but that doesn't matter). If the amount of rain is greater than TreeFactor, then the tile will have jungle or forest depending on the temperature. If the amount of rain is less than TreeFactor, then the tile might have forest (but not jungle), with the chance of forest being proportional to the amount of rain.
I added two additional constants, ForestModifier and ForestFloor. In my modification, the amount of rain is first compared to ForestModifier * TreeFactor. If the amount of rain is greater than this, then you get jungle or forest just like before. If the amount of rain is less than ForestFloor * ForestModifier * TreeFactor, then you will not get forest or jungle. If the amount of rain is between these two values, then you have a chance of getting forest (but not jungle) just like before.
There's an in-game option with three levels of forest: Heavy, Moderate, Light. Moderate is the default. The values for these new factors are 1.0 and 0.0 for Heavy (so that Heavy is the same as the current amount of forest), 1.3 and 0.25 for Moderate, and 1.7 and 0.35 for Light.
If you want to modify things further, TreeFactor is still there; increasing it will decrease forest for all options. Or you can modify the values for ForestModifier and ForestFloor.