In the Windows Task scheduler, for Opera's autoupdate task, the action looks like this:
C:\Program Files\Opera\autoupdate\opera_autoupdate.exe --scheduledtask --bypasslauncher $(Arg0)
, so maybe --bypasslauncher will work.
For doing that in a command prompt (for the right user), it might be something like:
"C:\Program Files\Opera\autoupdate\opera_autoupdate.exe" --scheduledtask --bypasslauncher
or
"C:\Program Files\Opera\autoupdate\opera_autoupdate.exe" --scheduledtask --bypasslauncher "C:\Program Files\Opera\autoupdate\opera_autoupdate.exe"
, but not sure.