I've worked out how to customize the Dynamics 365 Ribbon by using Ribbon Workbench and add an external url to a button, however what I would like to do is add a button with a link to the 'dashboards' entity. How can this be achieved?
There is an unsupported way to achieve this if you want to do it in Ribbon/Command bar button.
Like the same way, how you used the external url in a custom button, use the below urls for dashboards.
URL for All Dashboards:
CRM Server URL (for eg https://orgname.crm.dynamics.com) + "/workplace/home_dashboards.aspx?sitemappath=SFA%7cMyWork%7cnav_dashboards"
URL for Specific Dashboard:
CRM Server URL (for eg https://orgname.crm.dynamics.com) + "/dashboards/dashboard.aspx?dashboardId=[DashboardGUID]&dashboardType=1030&pagemode=iframe"
To mention, Sitemap is the supported way to use dashboard navigation.