So far, the only ladder racks I've heard about are ones that install in conjunction with tonneau covers or caps. I am not sure whether they are proprietary or whether they struck deals with various rack makers. I'm no expert on ladder racks, but most of them either install their own rail system or make use of the vehicle's built-in track system on trucks like Toyota, Nissan and Dodge from what I can see. Have you tried calling invis-a-rack and some of its competitors?
You know what the truck world needs? It needs a common specification, short-profile truck rail add-on for adding accessories like racks, tonneaus, caps and the like. The major manufacturers and parts makers should have an industry group whose job it is to meet and figure this out. That's how we get things like Wi-Fi, Bluetooth and most recently Thunderbolt 3 in the computer world.