I was making just the same script for my N82. If you look at the python docs in the camera module, you will see that these are the available options:
* exposure is the exposure adjustment of the device. Exposure is a combination of lens aperture and shutter speed used in taking a photograph. The default value is 'auto'. The following exposure modes are supported:
Sets exposure automatically. Default value, always supported
Night-time setting for long exposures
Backlight setting for bright backgrounds
Centered mode for ignoring surroundings
You can also check for the device capabilities calling the exposure modes() function. My N82 returns just the same as above. The problem is that my script takes 4 pictures, each one with one of this values but the resulting images only have 2 different exposure levels. I at least hope to get 3 (night, backlight and center) with the auto mode shoot just to test all options. And then another problem is that this 2 different pictures are quite similar, plus you can't use autofocus on python so you end with not enough samples for your HDR photo.
The end is that the camera module is not ready for this kind of jobs now, as it far from the real camera capabilities of some of the new symbian phones. I'm now working with the real S60 SDK that I hate but for a simple app like this I hope to have something very basic working by this weekend.