Maybe someone can help me out here. I remember seeing an ad in the last month or two (probably in EDN or Electronic Design) for a chip that allowed you to transfer data over a DC power supply line, I think up to
28V. It could have been by Maxim, but I checked their website and couldn't find anything. My memory is a little hazy, I don't remember if it gave you digital I/O on the remote end, or just let you access a few bits of memory. Either would work for me.
I have a potential application where I want to put an ID in a device that is powered off of 24V, and this device (or what I could remember of it!) seemed useful.