Dirt / Dust Shader, Light Interaction

I need some extra brain power to solve the last glitch in a floating dirt/dust shader.

Essentially, I'm trying to simulate a semi-opaque dust particle(s) that are illuminated (become visible) when backlit by a light in view of the camera.

I Have a successful front lit setup using a 2D PNG RGBA of dust specs set to the texture of a Transparent/VertexLit Material. (this is the standard Asset download from source, edited with Culling Off to allow 2 Sided texturing) applied to a Mesh Plane.

(Plane) <-(Camera) (Light) is working, but I need (Light) (Plane) <-(Camera) to yield the same illumination (ie map the light from the illuminated back side to the visible side of the plane).

Goal

Thanks and Cheers