I wore my zip off hiking pants the whole day (we did a day hike), and just brought a heavy running pant in my back pack, and pulled the running pants on over my hiking pants for the glissading. I wore my patagonia windbreaker which is water repellent, but not water proof over my dry fit hiking shirt.
Running pants were wet when I got down, but hiking pants under were mostly dry.
We summited on July 6, and glissaded down around 2.30 pm, very slushy snow at that time on that day.
Enjoy your trip!