Is there a way to do TRUE additive blending animations and layers without ridiculous workarounds?

Lets say i want a head to bob up and down. If i have a character taller than another, they will have differing starting points. So I'd want the animation to be relative if i want the functionality to work on both. Yet if i record an animation and move the head up and down, now i'm setting it's root position in the first frame. I ONLY want the relative movement to be recorded, ONLY the change from the starting point

Rigging and animation in Maya, 3ds Max, Unity is so wonky. Why do all animations assume a root position and rotation at all times? I could go into these animations and painstakingly drag every position to be centered on 0 ne property at a time and nullify the starting interference, then create a frame 1 pose of 0 for every animation property thus normalizing the key pose to zero for additive. An automated system that subtracted the start pose from from the end result from start to finish would accomplish this result automatically, or better yet, a system that simply subtracted the starting frames values from the get go, a sort of "additive animation mode", automatically saving the 0 value base pose at the very start, giving an automatic base pose to allow true relativity, followed by frames 1 to end frame with the actual relative animation.

Does a method like this already exist that I'm unaware of? Some areas of game dev are remarkably poorly documented...