At present there is NO summary data (eg average, max & Min) power output for a free ride period. In some of the training programs (4week FTP Builder) several of the structured workouts require a “best effort” free ride for some duration - it would be useful if after completing this element if the screen would display the average power you achieved for it. This would enable you to judge improvements in your effort from training.
While not a direct feature of Zwift, if you have a Strava account and sync your rides, you can do that type of analysis in Strava.