Well I have... But the problem with that sort of ribbon cable is soldering to the conductors. (I've intercepted the old round conductor ribbon cable (like the rainbow stuff), and I've soldered that sort of flat flex-PCB, but not flat flex ribbon. Yet.)
It doesn't matter where you intercept as long as it's in the single common dedicated wire of the LED supply - ie, +ve or -ve.
An afterthought - you might find that some of the strings drop out - especially if you dim significantly. In that case, PWM will be the solution.
jeunefred - still there?