http://www.minecraftforum.net/forums/mapping-and-modding/minecraft-mods/2524489-realistic-terrain-generation-rtg-realistic-biomes?comment=829

Can I also add to this a request to control how steep the slope has to be before it gets stripped of grass/dirt/snow? I'd really like to see mountains that had grass and trees on the lower slopes.

Yep, both of these requests will be implemented in #322. If you want to monitor the progress, just drop a quick comment on that issue.