Let's set some "ground" rules so to speak! Remember this banter is in the spirit of a discussion forum. Someone show me otherwise, but aside from the Lionel Acela are there ANY other consists that have a daisy-chain tether across more than 2 cars? Wired or wireless? So I'm suggesting some work, experimentation, mistakes, mulligans, do-overs, etc. are in order.
Here's what I was imagining...based on eBay listings as shown where you get a pre-wired male-female pair for less than 10 cents. I was thinking each middle car has a socket-to-socket cable that runs the entire length of the car. This cable can be fastened to the under-chassis so you don't even need to open the car! Again, this is the keep-it-simple alternative where the objective is just to get the directional lighting information from the powered car to the end car(s).
Then, between cars is a plug-to-plug tether to transmit the 2 wires to the next car...but to allow cars to be uncoupled and to allow cars to be facing either direction. The 2 wires in the plug-to-plug inter-car tether would likely be encased in black heat shrink or whatever to make it look like a prototype power cable. This tether and extra 1/2" or whatever to allow for bending around curves and what not...again just like the real McCoy.
The powered car and the end cars will have an electrical connection so the daisy-chain enters the interior of the car. The electronics to "send" and "receive" the directional information would be fairly simple...less than $1 in parts on each end. And some homework has to be done to accommodate PS2, PS3, etc. lighting.
Separately, if a 3-wire tether is desired. Then, yes, perhaps something could be done with the interior lights to have all the interior lights across a consist behave together...whether it be basic on/off or flickering in unison (if that's what a prototype did).
And it may well be one has to buy a crimper, loose pins, etc. so as to make exact length cables. But the point is these tethers should NOT be that expensive...