In any of the YZF-FX-WR models that I have seen, the red wired circuit is battery + throughout the harness. So long as you tap into battery + at a point with a large enough wire gauge to power the lamp (18 ga should be plenty), it ought to work, it would seem to me. You would obviously need a switch. I don't have a current WR schematic, but the non-EFI models ran the headlight direct from the stator off of AC, no relays involved.
Yeah, that is my plan.
Just wire it direct with a high amp switch or a mini switch and relay.