I've wanted to do this for a while now with an 80 gal (48x18x21) tank I've got down in the basement (currently housing a few Masdevallia spp. orchids)...but when it came to using it for frogs, everyone so far as said: "They won't find food and they'll all die." Whether or not that's really much of a problem, I have still been thinking about using it to house plants, just like you. And like you, I have been having trouble with figuring out lights to get the bottom light really well.
Halides are the best option--they put out more PAR and intensity per watt than anything else (even PC's...not sure what that one guy was talking about). But heat and UV can be a concern, especially if you plan on being at all exposed to the light being emitted. PC's are the next best option, but I still think it's going to be hard to find a setup that will still get decent light. You could probably fit a couple 96W Powerquads or a few 55W bulbs, but it's still going to be fairly shady down toward the bottom. It might just be that you use what you've got and work with the different microclimates within the tank. Lighting the tank from the side is an option...but I'm not sure how you would pull it off without it looking like a science experiment in your living room.
As for wood, I have quite a few large pieces I have collected, and was planning on securing them with some Great Stuff along the edges, and maybe cocos panels, etc. The majority of the background would be actualy materials, but just the gaps would be filled and securing done by the foam.