I'm sure it can be done with some SQL wizardry. A workaround to get you close would be to add a 'Multiple Object Chart' resource to the dashboard, choose the nodes / interfaces etc you want to analyze, and choose the sampling interval to be 30 minutes.
The data is automatically averaged across the sampling interval - so, if the average is > 75%, then the chances are that the threshold was breached the entire sampling interval (but not necessarily true). Once the chart is rendered, click on the chart and export it to Excel by choosing the 'Raw data' button at the bottom of the page. You can work out the counts pretty easily in Excel. As I said, it is not exactly what you are looking for, but should help you get a reasonable idea.