On the back of my discharge order it states that "a debtor may voluntarily pay any debt that has been discharged". I think, since the debt has been discharged and that the creditor had no legal right to payment anymore, you should have great leverage in offering a PFD. The creditor is no longer entitled to any payment so one would think that they would accept any payment they could. Haven't tried that myself however. Almost all of my creditors (except WAMU/Providian) just stopped reporting after our BK. I've also GW'd a few and they've deleted or revised what they reporting. I say go for it, it's definitely legal to do so.
If the debt is minimal and I could afford to, I personally would PIF. It was afterall a debt I was responsible for. For larger debts I would start at offering 30-40% and go from there if they don't accept.
I also disputed many of my accounts that were IIB and most of the OC's just deleted the tradeline. There were a few stubborn stragglers however.