Passive Ethernet Hub / Point to Multipoint Ethernet
The two problems is that while the devices might be turned off, there is still standby power. The ethernet cards can still be active at the base level. Which is why Wake On Lan works.

You could try a 4 pole, 3 position switch. This is the same type of switch used in radios, or more computer oriented, vga switches. Quality isn't always the best.




The better question is why is power consumption an issue? Can't you tap into the power from one of the devices?