Sometimes, on some forks, the bottom of the steerer has a hole clean through big enough you can stick your finger in. If you grab a flashlight, sometimes, about a half to 3/4 inch up, there is a hole in either side where the fork blade mates.

If you are lucky, you can feed the dynamo wire up the drain hole at the drop out. Then up the blade & out the hole in the top of the leg & out the bottom side of the steerer. You might find that making a loop with some 20 or 30 gauge magnet wire & inserting it in the vent hole from the bottom side of the crown so as to "catch" & extract the dynamo wire will help with your goal.

As you can tell, I'm not a big fan of drilling holes in things, but if you are lucky & have perserverance you may be able to at least run the wire up the fork leg. From there, just run the wire along a shift or brake cable & save yourself the trouble. Dynamo wires are small & easily broken enough that you'll be changing it frequently anyway.