You are making it harder than it is. You can't pump any oil overboard anywhere, well above about 1 ppm if I remember correctly. In the industry you hear all the time about magic pipes being found during port state control and other inspections leading to extremely high fines and prison time. For reference, a magic pipe bypasses the oil separator making you able to discharge anything.

Simply add another annex to MARPOL regarding discharge of sulfur and the industry will follow, penalties for breaking MARPOL are usually personal and not on the shipping company. Though, as always with the maritime industry the international manner makes any change slow.

https://en.wikipedia.org/wiki/MARPOL_73/78