I think I would like to do this, as I feel the extra light at night would be welcome. I do find the headlight a bit weak after my last bike, a Suzuki V-Strom. It had two 55/60w beams which both operated together (just like a car). I don't know why Yamaha didn't go that way - I guess it might require a little more alternator output, slightly heavier wiring and switches. Or maybe it was just that they didn't want to have to change the switchgear and such from the FZ8 for the Fazer 8. I guess this way we do have one unit optimized for low beam and one optimized for high (if you look carefully the reflectors aren't quite the same), but I'd rater have a full 110/120 w of lighting rather than the 55/115 that we get on the Fazer.
I see now what you were talking about - the left hand light only has a low beam bulb, and the right is only using the high, although the bulb has the low beam filament as well. I hadn't realized that - so the left hand low beam always stays powered. Is its power even routed through the high/low switch? Or only through the headlight relay? I guess I really need a wiring diagram for an FZ8S, rather than the one for the FZ8N (which I got as part of a downloaded service manual).
If I understand what you guys are saying, the low beam wire coming out of the switch is not being used? In which case, it's no brainer to hook it up, and doing so shouldn't result in any heavier of an electrical load on anything than when running the high beam. The switch and the wires from it to the headlight would be my worry otherwise - as to whether they were sized to handle twice the current as usual, but if the left low beam isn't going through it, it's not an issue.
Oh, and as to why there might be an unused low beam wire coming out of the switchgear: I suspect it's because Yamaha used the same switches, including the attached wiring, on both the FZ8 and Fazer 8. On the FZ8 that wire is connected in the main wiring loom to the low beam wire of the part of the loom that connects to the headlight. I'm guessing that on the Fazer the green on black wire in main loom headlight connector is instead connected directly to the headlight relay, same as the wire that feeds power into the headlight switch.