Obviously if the bankruptcy proceeding results in a pension haircut I would expect the unions to sue the state. They would use this clause in the constitution to say the parent (i.e. the state) is responsible for the debts of the subsidiary in this case Detroit. Bankruptcy law doesn’t seem to stop the creditors from going after the parent even once the subsidiary bankruptcy has been completed. I would guess that whatever the state court decides would be the end of it. The USSC probably doesn’t get involved because I can’t think of a way that Michigan’s constitutional pension guarantee violates anything in the federal constitution.
Now say the unions win and enforce Michigan to make up the debts of Detroit. That would force Michigan to raise taxes and I would guess that you’d end up with a referendum to overturn that constitutional guarantee.