This is probably way more information than you care for, and I am certainly no expert, but as I have spent countless hours researching the very premise of your post, I was compelled to speak up here for the very first time. I am a geologist and spend part of my time cleaning up the nasty nasty messes we humans leave behind, and spend the other part playing in the mud. I am acutely aware of the unbelievably toxic constituents we allow to be used in our every day breathing space. When I started making plans to build Girlfriend's viv, considering she is a desert dweller and that she requires such high temperatures, factoring in that I love her more than many people I know
, and capping it off with my knowledge of solvents, I was determined to give her the most toxic-free environment she deserves.
To address your query about PVA or wood glue, I think it is a really good product and I used it to reinforce the joints of my viv. However, as a "sealant" it is only water-resistant and that is a stretch and I don't believe it will hold up over time. There are stronger, more porous resistant types of wood glue, but these contain antimicrobial chemicals that I would definitely steer clear of. Liquid Nails, which is advocated by many people who have had plenty success is petroleum based and I would never use because I see on a daily basis what petroleum does to living things, in less than desert temperatures. I have tried to find the chemical components of Mod Podge, and have come up empty. The only information I can find is that there are no harmful chemicals. I think it is a good product for the use in question, but since you can't get it, let's scratch it off the list.
When researching stains, paints and sealants for the viv I came across "low VoC" products here in the US. Most of these are a sham. Because we have such weak regulation here (Europe and Canada have much more sensible regulation of constituents like ammonia) and because I am educated on the additives used in these so called "green products" I had to dig deep to find the products I would use. Please know that paints, stains and sealants that are advertised as "green" commonly contain formaldehyde, ammonia, heavy metals and acetone, just to name a few nasties. These poisons, under heat, offgas for years and years. I know you may think I am crazy but sprinkle some ammonia in your viv, provide 100 degree heat, and hang out and see how pleasant the experience is!!!
I fell in love with AFM Safecoat products, they use the innocuous propylene glycol instead of the commonly used, poisonous additive ethylene glycol. Their sealant will actually seal in offgassing from plywood glues, paints or any other noxious materials. It is made in the USA but here is their site
http://www.afmsafecoat.com/about_us.html. Since you are in the UK I tried to research some equivalent products and found Ecos. From what I can ascertain, they are good people, and here is their international link
http://www.ecos.me.uk/
Sorry for the diatribe, I am clearly impassioned about the subject. My viv has turned out "well" considering it is my first wood-working project and I applied the first coat of sealant on my faux rocks this weekend. From what I have read, five coats is appropriate. If PVA turns out to be your choice, I believe it to be the least toxic, if not the most resistant, and I would second that any day. Good luck from a fellow lover of dragons!
Dani