One Channel Digital Communication

You can connect together any number of pull-down outputs and a pull-up resistor with no danger of short circuit. There needs to be scheme that avoids two outputs pulling down at the same time, but that is harmless if it happens. A possible scheme is for one transmitter to be Master which sends a 0 pulse every 10 seconds. All the other transmitters are Slaves who know their distinct addresses so that Slave #1 may send something 1 second after the Master pulse, Slave #2 may send 2 seconds after,.... and so on. This example is deliberately slow because the data rate will be limited by the capacitance of the signalling wire, which we don't know.

The one-wire protocol device that KyranF mentions is clever because it manages to get its power from the same line that it occasionally pulls down.

This approach is common in industrial control systems such as AS-i.

^{simulate this circuit – 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. ;^)