You can see the hole I made with the drill bit, and the dried gas on the top from the test run. Next to it is a Verlicchi cable adjuster I had laying around with M7x1.0 threads. The threads aren't important, but the screw is, as it already is bored out through the center to accept the cable in its former use and the length was good enough to clear the cap and still have enough room to attach a hose.
I tapped out the cap to accept M7x1.0 threads and used the base of the adjusting screw as a stop on the bottom of the cap.
Attaching the fuel line vent was straightforward. I wanted to retain the adjusting nut to help secure the screw/vent in the cap, but it didn't allow enough clearance. To fix this, I just put a hose clamp on, and that'll keep the fuel line on and keep the screw from backing out with engine vibrations. I needed a method of securing the fuel line away from the tank in a clean manner, so I added speed-holes to the steering damper knob. Functional, but doesn't look out of place on a cafe bike.
The line isn't took noticeable, but it was pinching a little in it's current position. I needed to remove the kinking and had to raise the hose a little, which doesn't look as clean.
Not sure if I'll just replace this cap with a better, stainless, proper venting cap, or install an L pipe to allow the vent line to go straight to the front of the frame without having to arc it (as pictured).