This particular detail is very tricky and complex to resolve. What you can't see is that we have built a curb which the window system sits upon. Our drain pan and flashing system turns down the curb and under the stone porch. What also helps is we have deep porches with large overhanging roofs to reduce the amount of driving rain against the glass.

Thanks so much for taking the time to answer, though I don't have enough knowledge of construction to guess whether the process you describe would be considered an established protocol that most installers would know about or whether it is something you've designed or figured out that is original or at least specific to this site.

There are no real established protocols for this type of installation. We tend to call them "art projects" where the entire build team works together to resolve the construct-ability issues related to the Architects design intent.