treeform, I was thinking about this, and although the Rope class method David mentioned is probably the way to go there is actually a much simpler method to calculate the rotation for the billboard in matrix form.
You start with two vectors: L, the look (or forward) vector of the camera, and A, the axis calculated as the normalized difference of the two points.
Next calculate T as the normalized cross-product of L and A. This is the perpendicular of the billboard axis (basically A runs left to right on the billboard, while T will run bottom to top). Finally calculate P as the cross-product of T and P.
This will give you three unit length vectors which form the rows of the rotation matrix for your billboard, which I believe can be set manually and converted into a standard Panda3D rotation. Set the correct position and a scale factor proportional to the distance between the two points and you have a complete billboard.