Funny you mention it- I just came to the same conclusion: after doing research, I am scrapping the spray foam idea. It's chock full of synthetic chemicals and outgasses nastiness. Not to mention it expands with enough force to bend window frames and even studs. I think I'll be ok with the closed air between the double walls, and let the evaporation of the mist keep the temps down.
As mentioned before, the main reason you wont want to use pvc is that it will undo any control you have over the mist. it will cause the mist response to not be crisp- anytime the pressure is lower than 50psi or so (like when the pvc is pressurizing/depressurizing), the droplets will be too large and undo any attempts at finer mist (Imagine a nice fine misting -but at the end of the cycle spits big blobs of water all over the roots). This will be the same effect as pouring water on them to begin with. Plus your timer might be set at .5 seconds, but with pvc depressurization could double your timing to a second or more. As I mentioned- Cavadge dropped the pvc manifolds and switched to exclusive JG with reporting MUCH better results- this is why I asked if you really read the stuff, or just skimmed through it? ( I know- there really is so much to absorb and retain...

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I wouldn't have worried about the glass fibers because they would have been completely trapped in the epoxy- although not too sure how toxic epoxy is after curing either.
I had the same issue you are having. I was able to fit a regular PVC (it's actually the same as NPT) 3/8 male to 1/2 female threaded reducing bushing in the Accumulator which now sizes the accumulator output to 1/2 NPT female. After that I purchased a 1/2 npt male to 1/2" JG male stem piece then I used a 1/2 J/G to 1/4" jG reducing adaptor to size it down once more.
The pvc adaptor I just mentioned is located at any hardware store (standard schedule 40)- and I remember tree farmer writing that his leaked, but he switched to a schedule 80 adaptor and it stopped leaking- it looks like this:
http://www.grainger.com/Grainger/GF-PIPING-SYSTEMS-Reducer-Bushing-6MZ88.
And these 2 parts complete the transition:
http://www.freshwatersystems.com/p-1662-male-stem-12-cts-x-12-npt.aspx (1/2 NPT male to 1/2 JG male)
http://www.freshwatersystems.com/p-1627-reducing-union-12-cts-x-14-od.aspx (its 9 bucks, and the pic doesn't show the back side, but it is a 1/2 JG female to 1/4 JG female). These 3 mentioned parts will take the output of the accumulator down to 1/4" JG/PEX ready to pop your line into it.
If it makes you feel any better, this was by far the hardest thing to figure out about the plumbing. Took hours including reading plumbing threads to figure out that BSP was interchangable with NPT, and that CTS is interchangable with JG or PEX. What a confusing mess I agree, and I hated having to buy a $9 adaptor to fit the other 2 pieces together just because the correct piece I needed (3/8 npt male to 1/4 JG) was not a manufactured adaptor. Perhaps there is another way to go about all of this, but this was how I tackled it. Hope this helps you out man...