That function would be really nice, but, as Jon mentioned it is not really allowed/possible.
It sounds like you really want a wiki. You could create one huuuuuge course with numerous topics to replace your current course list. Then the tags would work. However, maintaining appropriate enrollment in this model is a nightmare. We have one course like this and ensuring people do the correct courses is a major PITA.
On the other hand, my company has started using Confluence which does allow what you are talking about. We are working on having the content on Confluence for general information access while using Moodle to develop a training plan and any required exams with appropriate links between them. Like I mentioned, Confluence has the content while Moodle contains the required objective evidence of completion. (My company got and started using Confluence without consulting ME
)