Communication over DC Bus at low data rate
This approach is common in industrial control systems such as AS-i. 







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Power and data on the same wire.

Here is a simple idea that meets your criteria and might even work!


Problem 1 is to make the DC circuit high impedance to your data signal. This can be achieved by adding a series inductor to each power feed and tap-off. You need to calculate the inductance based on the frequency of your data signal and how low an impedance your data signal can drive and still be readable by the receiver.
Problem 2 is to superimpose the data signal on the line. AC coupling should suffice for that. Again, you need to calculate the capacitor value based on the data frequency and load impedance.


Note that since the transmitter is most likely to be in the same location as the PSU that L2 could be omitted and the transmitter powered directly from the battery.

All loads on the line, including the receivers, will need a capacitor between internal V+ and GND to minimise feeding switching transients back onto the P+D (power + data) line.

You have a bit of work to do but judging by your question you have time but not money. ;^)