That's a really beautiful piece of driftwood, Tara. I wouldn't use any kind of preservative on it. Orchids are sensitive to some chemicals, and like to grow on rotting wood in nature. That piece will take years to break down so there's no imminent danger of it falling apart.
As far as attaching the orchids, I've used zip ties, plastic coated wire, and also strips of bird netting to tie the orchids onto the wood. I like the bird netting the best because you hardly see it. Other people use various glues like Liquid Nails, and things like old nylons. (who has nylons in Florida??) It's important to attach them firmly so they don't wobble around and that way they will get their roots attached to the wood eventually.
I have most of my Brassavola type orchids mounted. They seem to be less prone to black rot disease that way. I also have a small Laeliocatt, my little Sedirea japonica, some Tolumnias and a couple of Renantheras mounted. All seem happy. They do take a little more watering when they are mounted, as they dry out better on a mount. I spray them with water or fert/water nearly every day, and dunk them once a week in warm weather. Less when it's cool.
Sedirea tied to cork with bird netting, Tol. fastened to cork with zip ties, and Bl. Little Lulu on a cedar raft for a couple of years now, see how the roots have attached to the wood? I could remove the netting now, but I don't want to damage any roots.
