I have two Bostons that are very prone to reverse sneezing. What I was told on a Boston Forum and what has worked for me every time is to gently plug their nose so they are forced to take a breath through their mouth and self correct. I just cover their nostrils until they open their mouth to breathe. It usually triggers them to lick their nose as well and for whatever reason, that helps them to self correct and recover. It works every time for mine at least!
` Although mine aren't nearly as severe as the ones you describe. Mine have never needed medication for it. They know to come to me to get their nose plugged and it's over within a minute or two.