No. You can’t create tags that span projects right now, however ‘global tags’ is coming soon.
You can’t create tags that live outside a project because all tags (and data) must be contained inside a project.
This is by design; it helps us keep the conceptual model simple, and makes it easier to build features like project-level access control. We’re working on a way to introduce ‘global tags’ while keeping the conceptual model simple. In the meantime, you can use search to aggregate results from across projects.
If you name your ‘global tags’ the same across projects, you can use search to aggregate results from multiple projects in a single list. This isn’t quite global tags, but it does let you aggregate and quantify across projects.
This search query will return highlights tagged with Positive in the Interviews project:
project:Interviews tag:Positive type:highlight
This query will return things tagged with Positive across all projects: