Is it possible/can anyone explain how to transmit a USB signal through an "Ethernet over power" system.

OK, so we have seen "Ethernet over power" devices. In my home there is one with the "BT Vision" (its a UK thing). But I was wondering, does the input protocol of the signal in, allow you to send your own signal? I can understand if it can only send discreet digital signals, so analogue is probably out of the question.

I'm sure you are all wondering what I mean. Well although my idea is for something completely different, involving a camera, and a mains powered reloading BB gun, I will appeal to the SPY cam enthusiasts, who are often very clever with lots of ingenuity.

So you somehow manage to secrete a Spy-cam in somebodies stereo. How do you access the video? well there are 2 ways. one is to store is locally, this is limited by how bigger storage you can afford, and the limit of the device. The ones I know are normally limited at 4GB or so. 

The other way is to use my suggested way, locate the AC input of the Stereo, implant the AC part of the "Ethernet over power" onto it, then plug the USB device into the Ethernet part.(probably with some kind of mods). Then at the computer in question, connect the output to your computer, there is bound to be a 5v supply inside the stereo, but if not, there are plenty of 5v regulators available.

what does the community think?

There is a USB over Power system (similar technology to Ethernet over power) sold by a company called Icron here.

Regarding your camera problem, as a thrid option you could always transmit wirelessly.
I did find that website, but it mentions that it only sells these as apart of is other products, not as seperate systems. which is no good.
You're right... for some reason OEM's haven't put it into practice in any of their products for the home end user... there are systems using similar technology like the WiLife LukWerks Starter Kit Network camera here it is around $200 (then all you would need to do is wire the camera to the host device's power supply) and use the receiver and bundled software to monitor...

You could use an IP webcam (~$50) and plug into your ethernet over power system... or for around the same you can get webcam server for your existing USB webcam...

or if you want more control (like firing a BB gun)... you could use a mini linux system like the LINUTOP (which uses like 5W) and comes with VLC preinstalled... you could then stream your webcam via VLC over your power system or simply record to hard drive/flash drive...

Hope you find a solution that is more making and less spending...