You're spot on that (especially depending on the head and associated plumbing) a head mounted anywhere but amidships might be a problem. Wet plumbing
systems sometimes have their intakes clear out of the water
and sometimes the water
actually goes over the rim and onto the floor although that's another story in regards to poor plumbing
We currently have a composting head and too much heel angle will result in pee going where it shouldn't and bypassing the little safeguards that work at 0 or near 0 angles of heel.
My vote would be that if you can handle having the head amidships then do it, provided that you're not sacrificing too much on anything else. For us, it would mean having the toilet 6" away from where our heads are when we sleep in the vberth which is not a good thing.
And to toss in another curve ball, most vessels spend 90% of their time (or more like 99% in some cases) not underway where you might rather have something else more usable where the head would be.