DanC... Welcome to ECN...
I promise to be "gentle" with my response.
To what degree can insulated matting be relied upon for outdoor work on energized components?
Boy if you couldn't use rubber goods for protection outside, linemen would be in big trouble.
You should always follow the manufacturer's usage specifications for protective equipment. My suggestion would be that you not place an insulating mat directly on the ground. In fact, W.H. Salisbury (a major manufacturer of rubber goods) specifically states in its instructions that mats are not to be placed directly on the ground. Not because of the "dirt" and "moisture" but to prevent damage. These things are expensive, and they need to be cared for properly.
So, I usually recommend that they only be placed on a concrete pad that may be near switch gear or that a clean, splinter free piece of plywood be laid on the ground under the mat. A good plywood sheet good that could be used for this would be "fin-form" wood that is normally used for trenching and excavation shoring. It is designed to hold up well to moisture and because of its construction it can withstand a considerable amount of abuse.
The plywood suggestion is only my opinion, but, I have been asked this question before and that was my response. We have several people here that are lineman and they will likely "chime in" and give their opinion.
Here is a link to the W.H. Salisbury "blanket" catalog: http://www.whsalisbury.com/blankets/images/blankets.pdf
Information on the proper care and use of "blankets" is on page 7.
Hope this helps.