Maybe you can use some solid core, copper, thin gauge telephone wire that will be stiff enough to bend to a small "L" at the end but still be thin and flexible enough to feed through the first hole easily, down towards the other opening. You might have to jiggle it around to get it to eventually pop out of the second hole. You might also consider using thick nylon fishing line, but that tends to coil around too much inside the frame tube.
I remember using fishing line to pull my Avocet 20 computer sensor wires through the brazing vent holes on the top and bottom of my PSV's right fork leg to hide the messy wiring up front. It took a bit of time to do but nylon fishing line worked for me for that instance, mostly because the fork leg tube funnels down to such a small section at the end of the fork leg that the nylon line did not have any other palces to go but at or very near the vent hole. The wires are still in the fork leg and the Avocet 20 contnues to work to this day!
Good luck with the cable routing!
Chombi