I'm curious what others think about the impact, if any, of an employer getting a creditor to agree not to pursue "assets" in a rabbi trust in the event of the employer's insolvency. Would this separate agreement somehow make this a funded trust in the eyes of the IRS since the trust assets would no longer be subject to the employer's creditors?

It would certainly make me uncomfortable.

Moona's discomfort would certainly be justified. The one creditor that this exception would be negotiated with would likely be the biggest creditor, the one for which the participating employees have the most concern raiding the general assets of the employer and thus want this negotiated. And by eliminating such a creditor's ability to go after those modified Rabbi trust assets, you'd be decreasing the risk of forfeiture--likely below the substantial level.