An unplanned bivy is a most apropos topic now that winter is winding down. We were up on the Gerber-Sink route on Dragontail the same night as you. In fact, the night before, I met the partner at Colchuck Lake you were going to climb with from Portland.
In any event, my partner and I had completed nearly all of the lower portion of the route (nearly 1/2 of the entire route) when he became extremely ill unable to proceed any further. We then descended with three v-thread rappels and two full length (2 60m 1/2 ropes) raps from trees before darkness prevented us from completing two more rappels down the lower buttress at the toe of Dragontail back to the glacier.
So we tied in on a steep snow ledge and sat on our packs (which had closed cell pads)for the night. Hand warmers, dry gloves, extra socks, puffy, extra balaclava, food, water and stove all made for a tolerable bivy. We were surprised the next morning, when we rapped to the glacier, to be face to face with a hovering helicopter. I figured my wife called for a rescue (given the warm temps in Cashmere--she thought the worst had happened--avalanche)since I was now overdue, but the word I got when I came down was the sheriff was looking for a "lost hiker"--on Colchuck. Based on the recent rescue off of Ice Cliff Glacier, she insists that I invest in a Spot or PLD pronto.
Good comments from everyone about preparedness to bivy without adding significant weight.