I've done this on rare occasions without thinking about it, you know get erect and just push down on it. I agree with Big Al, you get a great workout to the pelvic floor muscles so when you do it kegel and the penis wants to rise against the resistence.
Wouldnt say these are anymore dangerous than other forms of Penis Enlargement unless you push down really hard, which could do something negative I would think.
When your pushing down the ligaments will be hit in a BTC-like fashion, as that position in hanging has the penis go in that direction on its route behind the cheeks anyway.
An idea would be to try and grip underneath the glans and when you push down, stretch out aswell and try to target the ligaments more. Use longer hold times and see how this goes. If it works for you, than go for it but adhere to safety concerns.