Hmmmmm let me think
Before render check the option in render globals :Render alpha Chanel
When your render view apears, and the image it's render try saving the image as Tif because not all files can save the alpha channel.
Import the Tif in Adobe Photoshop and after that, go to Chanels Folder, and control click the Alpha chanel, than with your right mouse btn on the layers folder on the image layer select layer via copy and .........
You get the whole image rendered butwith out the black background
It's tough to think but fo helping others ......