Maybe I am just too old, I would have thought this was a joke if I did not see manufactures hyping LEDS and putting the functional specs in the fine print.
It will be hard for ethernet to accomplish this. USB has the ability for the end device to provide power so the cable could steal some. Ethernet is extremely low power and if you used any of it to light LED you likely would cut the distance the cable could go. The cable actually uses voltage difference to do the signalling and the voltage is well below what most LEDS need to run. They would have somehow let the LED pull power without affecting the volatage.
There is PoE for ethernet but this is a active protocol and a cable can not turn it on. Even if the cable could...