Mannequin.js is not a general 3D pose modelling tool. It is mainly used to experiment with body posture and motion from a programmer's perspective. The Posture Editor is just a supplementary program and it is not intended to be a full-scale editor.
If you want to contribute to this project, please, ask in advance. It would be sad if you implement a feature that we are not interested in. The best place to discuss ideas for improvements is the Discussion section.