I can't think of any way that transport rules could currently achieve this (unless I'm missing something obvious).
You could setup a file policy in Cloud App Security (if you have it) along the lines of;
And apply governance actions along the lines of;
Alternatively, you could look into auto labelling options in the Security and Compliance Center to create policies that ensure an appropriate label is applied to all documents stored in your tenant. You will require AIP P2 or EM+S E5 / M365 E5 licensing to leverage this feature though.
Hope this gives you some ideas.