Camera projections with transparency don't render transparen

I'm probably doing something wrong here but I use this camera projection method in another widely used node based compositing app often.
As you can see in my screen capture, I am merely feeding an image with a poly mask defining its transparency to a camera that projects it onto some geometry.
That all gets shot with another camera that can't for some reason see or render the transparency, beyond what gets projected onto the geometry. I need the final output of the projection to adopt the transparency as well.
Any help would be greatly appreciated. Thanks.
As you can see in my screen capture, I am merely feeding an image with a poly mask defining its transparency to a camera that projects it onto some geometry.
That all gets shot with another camera that can't for some reason see or render the transparency, beyond what gets projected onto the geometry. I need the final output of the projection to adopt the transparency as well.
Any help would be greatly appreciated. Thanks.