Update:
Attached are three more billboard / camera facing material functions I’ve tried.
These three all yield the same results; they face the camera in all directions, which is what I want, but (unlike the material function shown in the first post) they do not work with the foliage painter.
Here’s a video clip demonstration the issue I’m having with the foliage painter:
https://drive.google.com/open?id=0B0eV5r9stGxac1U5cnJDTzZvS28
