Thank you for your help and clarification; I was able to successfully retrieve a given hour after I adjusted the GAQL query for my .net code to:
SELECT
campaign.name,
campaign.id, metrics.cost_micros, metrics.impressions, segments.hour FROM campaign WHERE segments.date BETWEEN '{startDateString}' AND '{endDateString}' AND segments.hour = {startHourToCaptureString}
Just to be absolutely certain then: there is no shorter duration than hour that I may get retroactively, correct?