Just had an email from botanist Glenn Rink. The plant is a Knowlton's Hophornbeam.
Glenn goes on to say: " Ostrya knowltonii, endemic to the southern Colorado Plateau, in canyons of the Colorado and Oak Creek Canyon area, as well as outliers east of El Paso in the northern Chihuahua Desert. This plant is included in Nancy Brian's special status plants of the Grand Canyon. It is fairly common in Grand Canyon, in the right habitat, which is mostly very dense, brushy, north facing slopes that are more mesic than surrounding terrain; places we humans tend to avoid. One odd location in GC is along the Kaibab Trail, on the Hermit Shale, in the open, right next to the trail, just uphill of the rest area in that saddle above O'Neil Butte."
I also updated my photo on my New Hance Trail triplog from last week: http://hikearizona.com/photo.php?ZIP=503452