Ah sorry i misunderstood, as far as i know any image format with an alpha channel will not save the camera background colour. if you save as a jpg you'll see the background. Only way i know of the top of my head.

TBH you're not meant to see the camera bg colour in a final render which is why it's not there. Why not just comp in any colour you want afterwards? It will take no time at all and give you a lot more flexibility especially if it's just a solid colour

My advice save as jpg if you must but personally i'd save out as a 16bit exr then comp in the bg afterwards. Someone else might know a way but it's not something i'd ever do.

Sorry I can't be of more help.
David