Julia, I think you provided some good links (and suggestions) for Anna to look at and contemplate about which direction to go.
Anna if you are thinking placement outside in the elements, I would suggest a good coat of Rustoleum for the cage before planting into it. If it will be in protected spot or indoors then maybe not. What light, wind and height exposure you decide on for placement of the cage might help to narrow down the types of plants you can consider.
If indeed the plant you decide on will stay potted in the cage and you want to soften or naturalize the look of the cage inside, there's a variety of moss (sphagnum(more spongy and moisture retentive) Spanish (more decorative) and reindeer (dyed, preserved for decorative)) that can be used to cover the pots. Or you may not want to cover a pretty pot!
