I love the magnetic anchor idea,....but like another mentioned here,...I would want the ladder to go a few feet above the roof line,...giving something to hold on to as I climb onto the roof.Thule now offers a telescoping ladder with a magnetic base to fix the top to side of van. It would make it a bit safer.
I recall some RV B-vans now come with this as standard equipment with roof racks.
Given the potential for failure of the plastic bits on these, I would be reluctant to alter one,...but curious to see a picture showing what you are talking about.Not sure if this applies to all telescoping ladder models equally, but I was able to make mine collapse several inches smaller by carefully removing about 1/2" of height from each of the rubber cones between the rungs with a razor knife, with no apparent ill effects. That meant that it could fit upright under the bed rather than lying on its side, a significant improvement in space utilization inside a van.