Thats a good a$$ question. It would appear your handle on the situation from a high low, left right, front back perspective is spot on.
Those would be the things I would reference if you wanted to get something to sit in the mix. Funny thing about the human ear and perception of space is its all trickery.
One thing I would ask is, why wouldnt filtering work (not just eq), but maybe some kinda of swepted filter, set your EQ bandwidth for the sound you have to tailor accordingly.
It would help to know what kind of sound to be able to say....nah that might sound unnatural. If you dont want to or cant use the typical methods you mentioned, then heck Im at a loss. Would love to know what others think.