Well through a good deal of time and effort, I did my due diligence and contacted several different tank manufacturers of PEX (crosslinked polyethylene) products, as well as plastic repair outfits, and materials handling companies. It led to two different solutions, and due to the nature of the injury, I narrowed that to the following.
First off, the incident:
Circular saw gouge into PEX tank, approximately 1" - 1.5" deep, approximately 4" - 5" long. The damage was not through and through, but grazed the interior portion of the tank. This resulted in a thin flap on the exterior, a 1/8" thick slit, and a very thin surface wall to the tank side which lightly pulled away. It was not easy to get into the main tank body, but there was room for air to escape.
I contacted the Gougeon Bros about a few of their products for plastics, and they steered me to the G-Flex line. The tech was informed of the problem and sent photos, for which he outlined a simple and thorough procedure to follow. The chemical makeup of the G-Flex allows its use in fuel storage settings like this, with a very minimal change in properties with fuel contact. He recommended the thickened G-Flex, using a flame treatment (key item), then fiberglassing in an oval shape.
First off, PEX is a low surface energy plastic, meaning that it essentially will not stick to anything, and nothing will stick to it. What Gougeon co. discovered is that by using specifically a propane torch, the non-polarized plastic surface, when contacted by the flame, oxidizes into a polarized surface. This allows materials to stick to it in a normal fashion. What was strange to me is that it doesn't require roughing with sandpaper. The following are direct instructions from Gougeon, included in their product packaging.
Flame treat the area:
Do not melt the plastic or deform/discolor it, just flame treating, creating the oxidation.
While the material is cooling, prepare your various items such as mixing supplies and fiberglass:
Next, prep the area for epoxy/repair by cleaning it with the provided alcohol swabs:
The material comes in tubes, so squeeze out the amounts needed on the provided square sheets (not a bad idea for quickfair as well) and mix thoroughly:
Apply as much material as necessary to fill the damaged area, then lay fabric into the epoxy. This was a little difficult through a thickened epoxy, but it did wet out with some coercion:
Clean and treat everything as you would any other layup, getting the fabric down tight and cleaning up excess. Make sure the fabric is properly saturated, without any dry spots:
Here is the final product:
That's what we have now. It holds over 3 lbs of air pressure without leaking, has cured hard but flexible with the tank (will be able to tolerate the shape changes with the tank's known expansion rate of 3%), and is not possible to pull up with hands or tools. In fact I actually tore up a piece of my thumb trying to get the glass to pull off.
I feel very good about this repair, and will keep an eye on the tank to confirm it doesn't fatigue with age. I have no question in my mind that this repair will hold and will serve for the life of the boat without a problem.