In all of your landing page templates you should include a text token called {{my.tags}} or whatever you choose.
Define this token in a folder at the very top of your marketing activities tree (I create a master folder that contains everything else). Input your tags as the value for this token.
Now all your pages that are local assets in Marketing Activities, regardless of template, will have this tag. You can update it in one spot and it will instantly update everywhere. You can also over-ride it locally in sub folders and programs if you like.