Sorry for the delay, I just realized I can run the moisteners on seeds and mycelium instead of cobble, and so I've rebuilt and simplified my original design. All it would take to switch to a mossy cobblegen instead of mycelium gen from this is to add in an extruder, chest and loader at some point on the track.
so - a wheat + seeds -> mycelium and mulch generator. Externally powered (I do believe you could make an exorbitant amount of farms and run the thing off it's own biomass for an internally powered system). Uses 3 moisteners and is supplied from an excess of wheat and seeds - I didn't do the experimentation for how many farms can fill 1 moistener with seeds and wheat, but i'm going to guess that number is 1 or 2.
The images are big and I don't wanna bog this forum down, so i'll be providing links instead of embedded images
Here's the system. The power supply is arbitrary, as long as it supplies 1MJ/t for each of the farm/combine layers. In the bottom left you see the actual railcraft sorting and moistening system. 3 loaders on top supply seeds to the moisteners, 3 loaders 1 layer underground and to the side provide wheat to the moistener's storage bank (wheat can't be input from the top and seeds are useless input from the side). Out the back of these moisteners which are 2 layers underground come buildcraft pipes which output to a crystal chest visible above the ground.
please let me know if anything is confusing