The problem with variable light strobing is that you're basically introducing a variable brightness level. Even if you slowly increase the intensity of ULMB depending on the refresh rate, as
blurbusters suggests, you're still looking at potentially large swings in brightness levels which might not look so good in practice.
I guess you could try and engineer every frame to a normalized brightness level, but I believe the amount of darkness that is added by ULMB would be variable based on how long it'll be before the next strobe of light, so because you don't know how bright to make the strobe until you know how long it will be until the next refresh, you have to add a frame of input lag into the equation, which is a pretty significant drawback that they'd probably never allow.
It just doesn't seem possible to me to combine Gsync with lightboosting as we know it.