I have oversupply and OAL. I too block feed. I have found two things helpful: 1) removing LO and allowing my BM flow into a diaper once my letdown starts and then resume nursing. They can get upset over this, but then may have a much better nursing experience. 2) pumping or hand expressing prior to nursing my LO on my full breast before nursing him.
:ita Allow the initial spray to flow into a towel or diaper. Also, try side lying or reclining back so gravity slows the flow, and baby can better control how much milk he is taking in. :)
Another option would be to wake baby to feed him earlier in the night (so breasts don't become overly full).
Here's some more info and tips:
I took my son yesterday to the dr. for something similar and she told me that the back arching is a sign of acid reflux. While mine has some other issues with laying down and spitting up and not the coughing like yours...you may want to call your ped if you're open to using meds to treat the issue. I happen to have Barrett's Esophogus which isn't passed down, but made me more aware of the damage that reflux can cause. Good Luck...