@gvmorley... I am not sure if you have upgraded properly to latest version of 6.x Dashboard Examples or not, because you should have seen version 6.6 also in supported version for dashboards. Since your screenshot just shows 6.5, maybe uninstall and reinstall if possible.
The reason why you are not seeing Drilldowns specifically for Splunk's built in visualizations is because new Drilldown options have been included directly in the UI Editor. Refer to attached screenshot for Latest 6.x Dashboard Example.
FYI if you want to explore new features of 6.6 you should try out Splunk Enterprise 6.6 Overview App from Splunkbase: https://splunkbase.splunk.com/app/3547/