I like the idea, though I'd probably prefer the portraits to show up at the beginning of an instance to show who you're playing with instead of the end with the Commendations. We already have the dead time at the start of every instance while we're locked in a ring waiting anyway so why not pop up the portraits at that point.

And, yeah, make the feature opt-out for those who don't want to see it but leave it turned on as the default.