You can get Butyl Rubberized ribbion for windshield/rear window glass repair at NAPA.
AT work we use Hot melt glue it comes in grey or black.
If I ever had to take a headlamp apart I would stick it in a pan and put it in the oven at 400* until glue is gooey, then separate parts and clean, being very careful not to damage the inside of the lens(it is not hardcoated)
I find that butyl rubber that comes with headlight works fine as long as coverage is good all the way along seam. The trouble is when/if u discover areas along the seam that have little or no butyl rubber (like aftermarked headlamps). I would imagine you would take some pure butyl rubber in a small pan and heat it up, and then apply it to headlamp. Wasting a screw driver while laying it out is probably not worth it - any tool gets full of almost non removable rubber.
It would definatly be an advantage if butyl rubber could be fed into a gun, but I am not sure how that would work considering its extreme stickyness - most likely tool could never be used for anything else.
I would go for no fume butyl rubber, if not it will fume up reflector lenses in headlight and make them foggy. I would suspect windshield butyl rubber to contain a vaporing liquid, so that a creamy consistancy can be had during install. Glass installers usually ask owner to wait 1-2 weeks get full adehiesion.
The question remains.. who has fume free butyl rubber?