I decided to fix my dunce move by just plugging the filler neck hole, and then storing the washer filler neck in the garage for when I need to top up. Took me a while to find something that would work to plug the hole which would be easily removable, seal off well and not fall in. In the end a worn out distributor rotor from a late 80's Toyota did the job nicely.
I also adjusted the aim a little bit and hooked up the Bi-Xenon solenoid wires.
HOLY CRAP. Even with just one headlamp converted, the lighting performance is amazingly improved. Night driving is so much better, especially dipped beam coverage. Distance, width, brightness - it's all good.
I decided that I will probably go in and shim the projectors down a bit to get the halogen high beam up where it should be. While the Bi-Xenon high beam coverage is very good, the extra halogen illumination at mid-range detracts slightly from long distance visibility. Can't wait to get the other side done, too.