Is there an IRS "deminimus" amount for reporting excess aggregate contribution refunds, which are paid after the 2.5 mos. deadline, on Form 5330? I thought that I heard an unofficial statement at one of the IRS seminars, but cannot find anything to support it.

well now, that is an interesting question. In a round about way there appears to be a de minimus amount.
under the APRSC program...see Rev Proc 2001-17
sec 6 .02 (5)(B) Delivery of very small benefits
"If the total corrective distribution due to a participant or beneficiary is $20 or less, the Plan Sponsor is not required to make the corrective distribution if the reasonable direct costs of processing and delivering the distribution to the participant or beneficiary would exceed the amount of distribution"

I see that the latest rev proc 2002-47 now has $50 as the minimum refund required!