Steve,
I also couldn't find anything about P. wrightii growing in U.S. My best guess, you have suggested, is that this plant was introduced in the region near San Marcos. Other than that, I would have to turn to you for information.
I got this plant from Error so maybe he can elaborate upon its source. It grows just like your descriptions. The internodes are very far apart from each other, and it grows extremely fast. New growths are semi-transparent. If you would like, I can try to post a picture for better confirmation.
This San Marcos River seems to be the hot bed for plants, Steve. How often do you visit this "store"?