It turns out that there is. It seems documented for some reason, but it WAS mentioned at least here long ago.
One bad thing about this (in addition to the worry that undocumented options may be more prone to change) is that it would probably get put right in the shortcut on the Desktop and/or the shortcut to do startup at login. There’s a manual change needed if the password ever changes, and the password is also more easily found.
The other plan of arranging for the tray icon to get the password from the database also has security issues, because the default database location is not accessible without answering a Windows security popup with an Administrator password. This can be fixed by putting the database in a less secured place than systemprofile or by weakening security in Duplicati’s folder (probably using command line tools not mentioned here before).
About Duplicati Security says why tray icon needs the password, and offers a maybe-still-future improvement.
Perhaps if @SamSirry could comment on security needs and technical willingness (preferably higher for the one who sets this all up, and lower for the end user or whoever might cause mischief), it would help guide me.