I think the best approach for looking at funding rules for pension plans is the governing structure of those plans and how they are overseen.
When the Ontario Expert Commission on Pensions wrote its report in 2008, it recommended that jointly sponsored plans be exempted from solvency payments, not because they were in the public sector or the private sector, but because they had a superior governing structure that allowed for better risk management and oversight.
I'm suggesting a principled approach, as opposed to just saying that because you're in the public sector you're never going to go out of business, and therefore you shouldn't have to make these payments.