I'm not at all tracking what you're saying.

All the discussions of the technique on gamedev speak of camera position relative to the billboard quad, not camera orientation.

Example: http://www.gamedev.net/community/forums/topic.asp?topic_id=411132

Specifically, a ring of billboards surrounding the camera would all be facing the camera, each with a different normal. In the algorithm you're using, they'd all be facing in the same direction, with the same normal.

---

Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)