Sean, if I understand you right you want to stain the ply then sheath it with epoxy/glass or polyester/glass?
You are right in thinking that neither stands up well to UV exposure, and you'll have to coat the resin with a good UV resistant varnish. You'll need to do some research and see what you can get hold of.
Without the varnish you'd probably be wasting your time, I'm guessing the UV index in Thailand is pretty high and will degrade your finish very quickly otherwise!
There are resins, both epoxy and polyester, that are UV resistant and you may want to look into those.
If you want to stain the wood I'd play it safe and use a water based stain, although I did read that Mark Bronkalla used an oil based stain successfully, he does recommend sanding with 220 grit paper after staining to improve adhesion.
See his comments and method here http://www.bronkalla.com/fiberglass_and_paint.htm