Thanks, Chocohead! I completely forgot that IC2.ini had so many configuration options, and it looks like I'll be able to tweak things just how I would like. I also now know it takes exactly 8000 heat to run the Fermenter, I was using a rough estimate before.
I had briefly thought about the use of reactor heat to run Fermenters, but I really considered the implications of it. I can see how it could sound better than a steam turbine.
For example: Using 8000HU from a reactor, the Fermenter produces 200mB biogas. Burning it in a Semifluid Generator (at 32EU/mB) it yields 6400EU for a ratio of 0.8EU per HU. This is better than the Steam Turbine's 0.75EU per HU, and at first glance that sounds like it makes the Steam Turbine obsolete.
It actually doesn't, though, since Steam Turbines don't require a constant stream of biomass. That matters for two reasons:
1) If the biomass requirement for Fermenters was high enough, a steam turbine could still be a preferable option for many players who'd rather not build giant farms.
2) There's an opportunity cost to using biomass for reactor heat. It would only actually be advantageous if it produced more total EU than BOTH using Steam Turbines for the reactor AND using that biomass "by itself" (I.E using a Fluid Heat Generator). Converting 8000 reactor HU to EU with superheated steam produces 6000EU, and heating the Fermenter with its own biogas produces 2400EU so that's 8400EU total. Using Fermenters to process reactor heat only generates 6400EU, so it's actually not a good idea. Increasing the Fermenter's biogas output wouldn't really change this, since it would increase both sides of the equation.
If they wanted to, the IC2 developers could tweak the Fermenter numbers quite a bit without impacting the usefulness of Steam Turbines.