Aleekat, thank you for the reply. I ran repair such as that by the glass guy, and he indicated it was critical that the repair be smooth and level, with no joints that could allow moisture in to create more rust. Just leveling it off with fiberglass as I was considering would in most likelihood be soon doomed to failure. In addition, this quick fix does not address the structural issues with significant corrosion present. The metalworking needed to properly fix this is way beyond my skillset. Closest quick estimate from bodyshop was around $1K, not including windshield and removal/installation of side windows, this for a 25 year old van with approaching 300K KM.
For now, the gap between the windshield and side windows have been cleaned up as best can, and fiberglassed over, so I can still drive it, pending a decision on what to do with it.