Use libksane
I'm not 100% sure but after I saw your tool on planet-kde I wondered why do you use scanimage where there's libksane to use sane directly?
Actually I looked into that. But as far as I understood libksane it is very tightly coupled to the preview widget and other UI things. I wanted to avoid that in for this project explicitly. Users should not need to fiddle with the scanner settings etc.
Based on that I found that using scanimage (which also comes directly from sane) a good compromise: The "expert" in the office figures the correct command line for the available scanner device once and all the users just use it and do not have to care.
Users should not need to fiddle with the scanner settings etc.
I'm using Skanlite and it doesn't expose this kind of setting, only the option from which part of the scanner the image is scanned from (tray vs flat).