>>2995984
Blomaga closed down a long time ago, so everyone scattered away to their own websites and blogs.
The communities, which seemed to mostly be for gatekept file distribution or enlarging the creator's self-esteem, also were shut down after the hack.
https://blog.nicovideo.jp/niconews/225559.html
...Though blomaga appears alive in some capacity when I check it, but the resources are gone.

As for why this method is not used, the reason must be that there's simply no need.
Either models generally have better weights than what I kept of the (rightmost) base or motions are mostly made with and for Tda models, and motion makers take care to avoid weird-looking contortions. I, too, could just adjust the rotation offsets in the motion for a smoother back.