"They told me that they coat the inside of their tubes with "something", but wouldn't tell me what, and told me never to repackage because it attacks virtually every kind of plastic, but refused to elaborate further."
I am not sure I believe the "attack" part. After cutting apart several Ben's 100% DEET bottles, I could find no evidence of any coating.
DEET is basically an oil. Generally, if WG will attack a piece of plastic, so will DEET. Generally this will "saturate" into most plastics. If this is what they mean by "attack" they should have said so. It is NOT usually destructive. Some plastics soften easily in the presence of oils. Some silicone/plastic blends used for coatings (on the Steripen Journey for example) literally turns into a gummy mess. Some plastics do not. PET, nylon for example soak very little. PET is often used for FUEL containers! HDPE is not effected by DEET...well maybe slightly, but not enough to notice. These are used as slides in guns where oil is often heavily applied. Hard plastics often fare worse. They loose integrity but will reharden as the oil disipates. Oil resistance was one of the properties needed to make sliding parts for items, sewing machines for example.
Note: I usually carry two bottles of DEET. I dilute both with WG. So, it is probabobly only 80% when I use it. Polyethelene or PET will not be disolved by this mix. Nor will my pants, usually nylon. Nor my tarp (silnylon.) Nor, does it effect the silicon coating. You thin silicone calk with WG to apply it. I am not sure about the coatings on the Gossamer Gear packs, but I had a bottle leak in a pocket a few years ago. It still works. Never really tested for waterproofness, though. I use a liner/compression bag so it wouldn't bother me if it did. Spectra line is not effected by DEET (well, maybe by lubricating the fibers a bit.)