Does anyone have any experience with the PBGC and waiver of penalties for underpayment of penalties? Is waiver granted freely or do they strictly interpret their reasonable cause guidance?

Do you mean "underpayment of premiums"?

I have two experiences with substantial problem situations. The first, about 10 years ago, they did not waive a penny, and the premium non-payments went back 20 years or so.

The second was corrected over the last couple of years. They waived most of what accrued after they issued a "where is the filing" notice and none of what had accrued before they issued a notice.

Of course I've seen many smaller situations. I would rate them fairly strict.