I am thinking of filling these voids with low expanding foam as
insulation -- product "Great Stuff". Besides being messy as hell, it
seems like the ticket since i can't find pitch-soaked cord. G

Don't use the red can, it will bow your windows. Even with the blue
can, only use a little, even then some will creep out into the finished
area and you will have to cut it off with a utility knife.