I agree with your suggestion. When and if the time comes in the future that the shunning parents are looking for money and assistance, I would say:..“if I’m rejected, so is my money. Please go ask the elders or the religion for money.” (Of course, this won’t work if the shunning parents are wealthy.)
If it were me, I would send my parents a letter now to that effect. This might get them thinking of the unintended future consequences of their shunning. I think a lot of people who threaten shunning are on their arrogant high horse and not thinking about what could happen. The religion isn’t going to take care of them.