I remember a few years back when I was in college and working a side job as a construction laborer we framed up a roof similar to this. The rafters were large I-Joists (TJI 560 equiv.) and were hangered from the glulam beam. The beam was probably almost 2 feet deep. I just remember I was glad I was cutting the blocking on the miter saw and not the one at the peak of the roof setting the rafters into the hangers.
With that being said I am wondering how often I-Joists are used in this type of application versus common lumber. I can probably add in an option to use them instead of common lumber. One question I have though is what to do at the other end, where they bear at the wall. I would need to study this further.