Well, I realize there's no built in option to make a HDRI render. But just as well as you could merge a set of photos into a HDRI in Photoshop, I'm thinking I could just as well merge a set of renders into a HDRI in Photoshop.

The basics of HDRI is having a set of photos, exactly the same scene, but with different exposures. I can set the shutter speed and the aperture size in my camera to do that. But is there any way to adjust the overall exposure in Carrara?