Can DOpus be used to scrub images and strip all metadata with a single click?
I think setattr meta * is all you can do... not sure if that'll get ALL possible metadata that some image programs might stick in an image metadata, give it a try.
I set that to a context menu command for JPG images but it didn't do anything. Do I need more than just setattr meta *?
Nope... else I'd have included it 
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How are you telling if it did anything or not? If you have the METAPANE open when you select the file, and then run that command from a context menu - don't expect the METAPANE to update automatically. It's always had some update-on-change issues... Click off of the file, then back onto it and you should see the metadata has been removed.
No, not using the pane. I right-click and open the metadata window. No changes after running the command...
Which values are not being cleared?
Well, from a quick test - the Date Taken field doesn't get cleared... and there are other artifacts as well:
- Screen capture type: after setting it to "Night scene" (which appears in the metapane drop down list TWICE?)... running 'setattr meta *' against it - rather than being cleared, Opus sets the field to "Standard".
- Subject distance: after setting it to "Infinity"... running 'setattr meta *' against it - again, rather than being cleared, Opus sets the field to "Unknown".
...I imagine there are other fields, these are just a random few I tested.
But @newguy: having the floating metadata editor dialog ALSO doesn't refresh on file change so - just make sure you're closing the dialog and then re-opening it after running the command; but certainly respond to Leo's question about what fields aren't being cleared.