I don't think turtles are as susceptible to metals and such in the water as fish are, so you might be able to get away with a bit more metal in a turtle tank then a fish tank. I'm not certain about that, and could be wrong, also it's not something I would do.
You could also use nylon machine screws - either drill through a piece of wood, and put on a nylon bolt, or you can tap the wood itself and put the machine screw directly into that. That's how I secure my driftwood to slate, and it works pretty well. Not something I'd recommend to someone just looking to anchor one piece of driftwood due to having to buy a tap, but for that many connections it might be an option. I already had one lying around, so that's my excuse.
The regular expanding polyurethane foam gorilla glue is also safe once cured, and some of the dendroboard folk have built their backgrounds with it, so that's something you could use to secure it if you go with the drill and dowel method.