This is common for randonneur bikes. If you do an image search for "dynamo cable routing randonneur", I'm sure you'll find plenty of photos.
For the rear light a hole is normally drilled in the down tube near the head tube and on the underside of the bottom bracket shell. The cable pops out of the BB shell and is then fastened on the inside of the fender until it reaches the light.
For the front light, you can run the cable from the vent hole near the dropout to the vent hole near the fork crown and then run the cable along the inside of the fender until you get to the light. It depends however on where you'll be positioning the front light.
If you decide to run cables internally, I highly recommend adding cable terminals between the bottom bracket shell and the rear fender and between the fork crown and the front fender. That way in case you decide to change the position of the lights, the terminals act as quick release disconnection points so you don't have to mess with the cables that run inside the frame and fork.