So I found a way to do this – its a bit roundabout but it works.
First you have to filter out the tasks you want to display. I did it by creating a column/custom field (text1 for example) and called it Work Group. For you, add in this column HLT (high level tasks) on the tasks you want to display. Right click on the header and choose the “Group” option, click on the filter icon and uncheck “blank”. Now you have the items for your display.
Unfortunately what I had to do is copy these tasks into a new project file and then select Calendar in the View section on the Task tab. And there it is.