Are there any known occurrences where poly or blends have melted and/or caused injury when worn under level 0 ppe with a panel labeled at <1.2 cal?

With all that is known about poly type blends and where they will melt and adhere to your skin, why would anyone doing electrical work or operating equipment where there is the possibility of an arc flash even consider wearing a poly blend?
I thought this was a well known practice not to wear poly blends.

Yes. Lots of tests and real life examples. Matthias in Florence, NJ had his hand severely burned UNDER an aluminized fire glove because he wore a pair of nitrile gloves under the aluminized gloves to keep his hands clean. One day the aluminized gloves were tru, y needed due to an oxygen lance gone wrong. The aluminized gloves survived and his hand would have survived if he didn't wear the nitriles. Same thing has happened in arc flashes, welding accidents, to fire fighters, etc. It is a very well known issue.

The real problem is that 1.2 cal/cm2 point is impossible to define. A 1.2 cal/cm2 calculation means the AVERAGE on the surface of the sensors in the IEEE 1584 testing in for the 2002 version was 1.2 cal/cm2. It could be more though at some point so, the melting clothing is prohibited when a worker is in the arc flash boundary. The 2015 NFPA 70E drops HRC 0 but keeps the requirement for no melting clothing inside the arc flash boundary. It is likely that this distinction will lead some to believe that it is safe to operate equipment with poly-cotton clothing worn. This will not be the case. The only way poly-cotton will not hurt a worker is for the worker not to be in an arc flash or for no more than a calorie or so to hit any part of the clothing. This is never certain in calculations unless you are well outside of the arc flash boundary AND all the calculations are correct. Companies really struggle with this but it is usually for operators. I recommend having them wear 100% cotton short sleeved shirts and provide an arc rated Kevlar sleeve and an arc rated glove for the HRC 0 devices. This protects the arms and hands but allows a common clothing type to be worn with little cost to the employer.

Get a copy of the latest Westex CD. It shows cotton with a much hotter flame temp - food for thought.

One of the first cases I investigated was a 3-5 cal/sq cm event where the arc ignited a poly blend polo shirt. All the burns were associated with the shirt.

One thing that still surprises me though. I believe I saw a reference in NESC that rayon is an acceptable nonmelting fiber. Is this correct?

Also I have been noticing that some FR rated fabrics contain nylon which is a bit of a surprise. I suppose as long as it passes the test it would be OK though.

I have also seen several references that rayon is acceptable. Rayon is manufactured from naturally occurring polymer (wood fiber) and is considered a semi-synthetic fiber. Perhaps that is why it is acceptable?

Rayon is cellulose fiber similar to paper. It will burn, but not melt. Synthetic versus natural has nothing to do with it, the question is whether or not it will melt into the skin when exposed to an arc.