The main structure can be wood, as long as your wiring has the correct insulation and you pass heat tests (I think).
UL has a program called "preliminary investigation". They will look at your product and documentation and tell you what you are missing and what tests you will need to pass. If you are 100% sure you will UL, it only adds a few percent more to the whole process, but can save you a lot more avoiding failing a test and having to start over. I'd call your local office and ask about it.
"There is hardly anything in the world that some man cannot make a little worse and sell a little cheaper." - John Ruskin