A hitch rack is a fairly good aero option, although it's much heavier and less aero than a trunk rack, and it'll obscure the number plate.
I'm sort of up against the same thing, since I'll need to support our distance rides this summer. I guess my current plan is to put on my 5-bike roof rack for the shorter rides and just throw a Blackburn Super Cycle Shuttle trunk rack in the car for ones where I'll be on the expressway a lot. The roof rack should be much more of an FE hit at high speeds than lower ones.
I'd stick with a trunk rack that can be adjusted for the hatchback similar to the Blackburn (I don't think it's still being made). If you want a project to work on that will make it more convenient, you could consider a cantilevered balance weight mounted to the interior of the hatch so you can use the hatch normally with a bike and rack installed.
One other option may be to construct custom towers for a roof rack that can be mounted to the hatch and point the bike down with the front wheel off. I expect this would probably disturb less airflow than a conventional trunk rack, and it'd be a bit more secure and convenient.
If you want to make it really efficient, figure out a way to put the bike in a boattail