So you are telling me you want to put a 300+ lbs load with your humerus at a negative 45 degree angle with the only thing holding it in place is a strip of tendon? Go for it man. Remember you used momentum to get it past the bottom half. It's not safe. You can only get away with it for so long.
Honestly I cannot comment as I do not do these but i will start but just a few thoughts:
If that was the case would you just not use lower weight but fuller ROM
Also if you are seated I do not think you could use momentum?
Like I said I have not tried these but i will and get back to you man!