What do you think that post effects are manipulating besides timing based ones like reverb/delay?

I see what you are getting at but there is no true equivalent to reverb/delay in the physical behavior of resonances in headphones. Resonances affect the FR in an exact 1:1 fashion to the point that one can be calculated from the other.

A resonance at a certain frequency causes a peak at that same frequency. Or does the peak cause the resonance? I dunno, they are inseparable.