Pests would be a problem with any mount material picked up outside. Boiling mounts before use kills off pests and disease.
As for moisture retention, not many mounts retain much moisture.
I looked up bamboo used as mounts for orchids, and it seems that some people have done it, but they apparently use bamboo slabs, which I would take to be an extra large cane cut in half. The inside of the cane is rougher I think, so the roots have something to grip on, unlike the smooth exterior.
Here is a photo of John's ( jkofferdahl) Phal bastianii mounted on bamboo. I hope he doesn't mind me posting his picture!
P. bastianii - Orchid Board Galleries
If you have a really small bamboo I suppose you could mount things, but it would have to be mini orchids, or else the mount will be out grown very quickly.