After changing to 8-bit mode in the AGS editor, updating the default to 'Software driver' in the 'Default Setup' tab, recompiling, it then seems to remove the option to pick 'Software renderer' from the Winsetup 'Driver:' drop down box. Changing back to '32-bit (true-colour)' mode and recompiling seems to restore the 'Software renderer' option to Winsetup.