Ok sorry for the lack of documentation about that feature
Here is how it works:
If you want to launch SainT in "autorun" mode, just add "-autorun" option in the command line. Just after, specify a name of a .INI file, wich is the configuration file used. ( Just read saint.ini for more details, it's quite easy to understand). So the best way to launch a specific image disk is to generate a custom saint_autorun.ini containing the right image disk name in the DriveA key.
when running in auturun mode, F12 key simply exit SainT ( you never see SainT user interface, just use the right saint_autorun.ini to set all options you wanted).