Well, it looks like there must be no cable made to do this. Even the case manufacturer has pointed me to an add-in card for $30 more per build.
It seems that I will have to make my own cables for this. Why isn't anyone doing that now? The parts in quantity would be of minimal cost. It should cost about $5 for one complete adapter cable. It will cost me more than $15 for the parts, per case, at these low quantities.