I recently switched to trekking/butterfly bars and have been trying to come up with a way to attach a mirror. I finally figured it out this morning.

I sacrificed a right-hand bar end from a set I haven't used in years to attach a Mirrycle MTB mirror, cutting the curved part of the bar end off and omitting the 2nd arm of the Mirrycle. This isn't the finished product because I'm holding off taping the bar until I'm certain I have the positioning just right.