Peter,
At best, the "warranty" would make the insurance company a fiduciary for a very limited scope of responsibility, usually due diligence selection of underlying funds (exclusive of wraps). In each that I've read, the role of the plans investment manager under ERISA 3(38) was held by a party outside the insurance company, usually the sponsor.
I'd question whether it would be possible for a sponsor in this situation to make a claim under the fiduciary warranty without subjecting him/herself to the possibility of a fiduciary breach claim. I think this acts a a pretty significant barrier to making a claim.
I share your curiosity about the experience though and hope that someone on here can share a story. The other important thing to consider is the warranty is only as good as the guarantor...even if well intentioned, they may not be able to pay depending on circumstances years from now.
In my opinion, it's just a marketing gimmick. There are real 3(38) managers in the marketplace, but I'm confident they are generally averse to the insurance companies platforms.
Mark