Hi Natalie's husband! If you're planning on making the structure that way, best bet is to use pvc pipe. You can run your irrigation system through it. Metal is bad for anything like this where humidity and water is a factor. Besides leaching of toxins, you run the possibility of corrosion, even with a coating on it. It's amazing where water can get to. Just ask the reef folk about the weird places they have cleaned salt creep from......lol
Another option would be to hide an irrigation system in the canopy. You can run a system like did with my madagascar tank pretty easily and inexpensively.
Also, other options on the tree coming out of the island are use dried oak branches instead of the ash. Or you could plant something live there such as a ficus ;'benjamina' or a tree fern. Growth would be slow, but as you grow, mosses can be added as well. Just a few ideas for you
one thing you will find with this forum is there is rarely ever a shortage of ideas