This would be a great solution if it worked. Unfortunately there is no way on earth that it ever will.
I think the confusion possibly comes from the use of the term "ethernet" to describe a cable, when in fact ethernet is a type of data packet that is sent down a cable. Because in 98% of cases this cable is CAT5/6 etc, people use the term "Ethernet cable" to reer to anything with an RJ45 plug on the end.
Materlink wil run over a CAT5/6/7 structured cabling system (as will telephones, HDMI, Scart, component, VGA, RS232 etc) but since none of these are using he ethernet protocol, they will never transmit accross a link made with these home plugs which simply send ethernet packets over another medium, in this case the mains cabling in your home.
I treid to keep it fairly clear but fear I may have failed and confused people even more.
Anyway, the thrust of it is that Cat5/6/7 cable, and ethernet are 2 entirely different things, and as such this idea will never work