You're right on track. You'd have to pick your plastic very carefully to make sure it could handle not only the heat, but the temperature cycling as well. Aluminum is the safest choice as the nearest mating surfaces are the same. You'd still have the benefit of at least 2 outlet gaskets instead of one for further isolation, although not much.
I haven't dug into it enough to see where the lower intake passages are fed from and go to, but I'm going to bet that the rear hole feeds up into the SC and TB, then back down into the forward hole, and on to the front head. Have to keep that path open.
BTW, now can everyone see why I meshed my cover and added ramair cooling?
Here you can see the yellow o-rings marking the passage locations: