Sending a clock over low voltage DC
You could always do something like the following block diagram:







simulate this circuit &ndash; Schematic created using CircuitLab

I'll leave the details on working it out to you, but simply putting filters where appropriate and adding your clock to the power line should do you fine.

If your circuitry on the receiving end is sufficiently tolerant of input voltage fluctuations, you could even leave off the low-pass filter for the power entirely, though it would increase noise in the circuit and I don't recommend it.



As the asker has said they've already worked out this much, I'll give a bit more information.

On the receiving side, you should use an LC input low-pass filter before feeding the voltage into your regulator. I say LC because that gives you lower losses; RC will work as well but provide a harder current limit and will waste some power. An RC filter would be my choice for the high-pass filter, however, since you don't need significant power going through that line.

On the sending end, adding the clock on top of the supply voltage can be done with a simple LC filter like this:







simulate this circuit

There are, needless to say, better ways of doing this, but I think you'd have a hard time finding a much cheaper or easier way of doing it short of using a resistor instead of an inductor. Again, the choice to use an inductor is to limit power loss. Putting resistors in series with your power source is usually not the best idea.