I worked to make the mid-edge constraint available which will depend of the SubShape code and I had some small problems but I will fix them tomorrow. Easier to be done will be middle of rectangle that I will also do it today. There are still some remaining missing functionality and I hope that everything will be done over this weekend.
There is also an UI concept that will expose the solver sensitivity in status-bar for easier setup using a slider. This work is done by our contributor Cristian. We don't know if this is the best way, but at least we explore this area.