How much space do you have between the towball and the tailgate of your truck at the moment?

Perhaps you can make up an adapter plate that will bolt on between the tongue and existing towball and will project forward toward the truck just enough to allow you to fix a mounting bar on it (off to one side would work too, but you'd have to be careful of your turning circle with a trailer on there as well).

Yes, it would raise your towball slightly, but 10mm would be negligible (whats that ~ 3/8"ish), and more than solid enough in mild steel to mount a T bar for the kayaks onto.

Seems to me that the simplest (and probably least expensive) solution would be an ordinary rack on the roof of the truck, and a T bar on the hitch bracket thing.

: I want to be able to still haul a trailer, so I'd rather not use
: the tow hitch for a mounting bar, but some other type that would
: fit outside the cap might work.