Last night, when it's around 5:30AM that I was just about to sleep while trying to avoid thinking some numbers and expressions echoing inside my brain. Then suddenly I came up with an idea! An obvious, and simple way to rig the spine just the way I was trying to do with expressions over some days. It was so exciting really. I almost jump out from bed if it wasn't dawn, cold and scary. :) Anyway, I just tried it first thing on this morning and it worked pretty pretty pretty pretty good. All I've done is making a point helper at pelvis bone, position constrain it between neck (which is also stretchable) and hips control object and make it "Keep Initial Offset" so that it stays in it's "default" place. Now I can link 2nd Spine IK's helper to this point and it stays between middle/hip area no matter how much I stretch the spine. I think I still need to use expression for the upper part of the spine though. Not seem to possible trying same thing twice in one hierarchy. But it's not big deal anyway. I could just move on...