Data over a 5V DC power connector
You could think along these lines: -







simulate this circuit &ndash; Schematic created using CircuitLab

The data you send needs to be DC balanced i.e. something like Manchester encoded or scrambled. The amplitude of the data needs to be significantly smaller than 5V - maybe 1Vp-p.

You'll need a comparator/data slicer on the data out signal to recover the amplitude to logic levels.

L2 and C1 form a 1600 Hz low pass filter to smooth the DC recovered - this means your data rate is best as high as you can feasibly go and with Manchester encoded data there will be no content at DC up to the data clock rate  - whatever frequency that is it ought to be at least 30 times higher than 1600 Hz i.e. approximately greater than 95kbits per second. If you can send M-encoded data at 1Mbps then that would be a lot better.

If your data is intermittent then send a preamble of 1s and 0s to get the data slicer aligned correctly with the data.