I have done quite some research to accomplish comments under ECK entities, but all the tried solutions and modules appeared to hamper or be incompatible.
The idea is to create an exposed field for directly entering comments. It should allow for multi-level commenting (replies to comments).
I will update this question with the tried solutions. If you have actually tried or implemented a commenting solution for ECK entities yourself, please post your answer.
If you have suggestions but have not tested it yourself, please post it as a comment to this question, and I will try your suggestion.