After staring at the engine from many angles on several occasions, I decided to stick with the original location for the oil return fitting. The reason is that the fitting on the accessory housing pictured above in this thread is in a tight spot and appears to be a different size, so there would be cascading changes to my plan for the oil separator installation.
I removed the exhaust headers from cylinders 3 and 4 to allow that part of the exhaust system to drop down a few inches, then removed the intake pipe from cylinder 1 and installed the return fitting. It went smoothly for now, as long as nothing decides to leak on me later.
The only real hassle is that I need to get some more MS35333-41 washers since there seem to only be 8 in the RV-14 kit and I know you're not supposed to re-use those.
Onward and upward!