We're planning to add this. Unfortunately we'll still be in the same situation once we add it. Most listings won't show manual vs. auto because we haven't had it. But requiring it for new vehicles will help, and a certain percentage of existing users will add it.
This is a good idea. I also agree on the real world data here. It influenced my last car buying decision and I am happy to report that the corolla is getting the mileage expected based on what was reported here and not the EPA estimates.