There is a Set Privacy addon tool. Unfortunately it does not work with Attributes.
I too want all of one Attribute Type to all be set Private; Social Security Number. I try to remember to set the SSN Attributes when I create them but every few months I run what I describe below and there is always 5 or 6 records that I missed.
I have tried to hack the code so that on save, Privacy would automatically set for that type. So far no luck. I am not a coder and can only tweak what someone else can dream up.
To do a bulk Privacy set, you would need to edit the xml backup file. The easiest way is to create a .gramps (XML) non-media Export. Make sure there are no filters placed on the export and uncheck the Use compression in the Export options. The file created, even though the extension is .gramps, is an XML file and can be edited.
Be Careful. I use only Search and Replace when editing so I do not corrupt the file.
and Replace with:
<attribute priv="1" type="Cause"
When done, you create a new empty family tree within Gramps and import the altered file.