Can ExtendedReports report on CiviDiscount usage?
Yes - you can now! 
See https://github.com/eileenmcnaughton/nz.co.fuzion.extendedreport/pull/148

Adding field label from the civicrm_line_item table allows admins to search (i.e. filter) for Contacts, Contributions that have had a discount code applied to them - in the standard report environment so Actions (exports) as well as Add to Group are available out of the box.

The screenshot below uses template civicrm/report/price/contributionbased - but any ExtendedReport will now have line label as a field/option for you to specify what discount code you'd like to search for.