I suspect that there won't be holes where you want to run the wires - as in, you won't be able to thread a cable down the vertical tube and along horizontal ones. When they made the railings, they'll have only drilled holes where they need them for fixings, and if galvanised, where they are needed to allow air out/in and molten zinc in/out during the dipping process.
So the best you are likely to be able to achieve, will be to run a cable down the vertical tube, and then clip it (either to the railings or the brickwork) go go horizontally.
As for fixings ...
If the lamp will mount onto a piece of pipe or (metal) conduit, then that would be the way to do it. You'll need a plug/adapter with hole in the middle to take the conduit, and square or round on the outside to fit inside the railing upright - then just drop the assembly into the top of the tube. Could be done with wood, or as suggested, could 3d print it.
Enhancements could include extending the tube upwards to lift the light fitting - but then it will need something to stop the tube sliding down the adapter, or the adapter sliding down in the railing tube.
Alternative, but not as good for aesthetics, and needing care not to damage the cable ... Drill a couple of holes in the railing tube, and drill/tap matching holes in the tube in the bottom of the light fitting. Slide the tube down into the railing, and secure with a couple of bolts/screws - taking care to use bolts that are short enough not to damage the cable inside the tube.
this is what I want, easy with photoshop, it will light my drive and patio and also the path to house, at the moment there is a square hole in the railings for a spigot to be dropped into, around 1¼" square, so wires can run inside the railings around the wall to where old lamps were, got the JB ready to connect, but wanted railings fitting first.
