What signal types are safe to use with a return ground path for microcontroller switch modules?

I’d like some input (brainstorming) on just ONE aspect of a project that I’m undertaking in my home.
This background is long, but it puts my requirement in context, so…
I’m the second owner of my house. The initial owner/builder had several electrical circuits with 3-way (3W) and 4-way (4W) switches; switches for the lights in hallways, basement, garage, outside lights, many in places where you can’t see whether the light is on or off from the switch location, and even some multi-switch circuits to control electrical sockets.
So this is what I want to do:
I've designed a module that is to be attached to the 3 leads (3W switches) or 4 leads (4W switches) of each switch in a circuit. The microcontroller-based module goes in each gang box behind the analog switch (they do fit). An LED coming from the module is positioned in a hole drilled in the wall plate of the switch. The LED indicates when the load has voltage (measuring current flow would be easier but that wouldn’t work for the switches controlling sockets unless something was plugged in and switched on).
When a light is on (or an electrical socket has voltage),
(1) only the common lead on the load side 3W switch “knows” that the light is on – if that lead has voltage, voltage is being supplied and
(2) when light is on, the unused set of traveler wires are connected together, are not energized and run from switch to switch for the entire run of the circuit, and this connection is isolated from everything else (I can provide a drawing to show this if necessary).
The module behind each switch already determines which traveler doesn’t have voltage and if the 3W load-side switch, it will send a signal on that traveler and if not the  3W load-side switch, if will try to detect that signal – successfully detecting that signal would mean the load has voltage and cause the local LED to illuminate.
So I’d like to use that series of unused travelers to safely send a signal from the 3W load-side switch to all other switches, but as far as I can figure, the return path has to be ground.
Brainstorming Question: What sort of low voltage signal (AC?, DC?, voltage level? something else?) can I safely (probably continually) send on the isolated set of unused travelers when the 3W load-side switch common lead has voltage, that can be detected by every other switch in the circuit attached to the isolated set of unused travelers, and for overkill, detectable as much as 150 feet of 14AWG wire away?
Secondary Question: If you happen to know, if I want to commercialize the idea (realistically not going to happen but...), will the solution pass UL scrutiny? By the way, if a circuit only has two 3W switches, I could swap out the 2 with an existing commercial solution (for a lot of $ per switch), but nothing out there can handle a circuit with 4W switches.Thanks.

I'm the OP.Here's a short version of the question from my initial post, slightly modified but the concept is the same:
I have a 150 foot section of 14AWG wire in the walls of my house.  I'm only using one lead, e.g., the white wire.  It doesn't have any mains voltage on that white wire.  I want to create a circuit using the white wire and electrical ground as a return path.  I want to generate a signal from one end of that wire such that using a ground return path is safe (and secondarily will be approved by the UL if ever I go that direction).  Whatever signal I use (AC? DC? voltage level?) has to be  detectable at any point of the 150 foot white wire (insulation stripped and tested on the bare wire).
What sort of signal can you recommend that I use?

Again, the original poster.
I used an online Voltage Drop Calculator and it appears that either AC or DC can easily travel the 150 feet with the low voltage, low current that I'll be using, with minimal power loss so my only question now is one of safety (electrical guidelines, UL standards).

Is there any reason NOT to use a low voltage, low current signal with the house's electrical ground as the return path, both at the beginning where the signal is generated and at any point on the wire that I want to test for the presence of the signal (insulation stripped and tested against the bare wire and ground)?
Is there any reason that a low voltage AC signal would be better than DC?

John,An interesting question. Too bad I don't have much time to think about it.
You might want to take a look at how the X10 protocol injects the signal onto the AC wiring for ideas.
https://en.wikipedia.org/wiki/X10_(industry_standard)

Thank you, Elizabeth.
I've since posted 2 revised (and greatly simplified) questions that are "awaiting moderation", apparently because I'm a new poster.  Once posted, I'd very much appreciate it if you could come back and take a look at them.
I'm familiar with X10 and it won't work for me.

This is the OP yet again...
This might be really simple and I ended up posting to myself, but the process did cause me to do more research and more thinking...
One question (and I'll probably figure this one out):  When a light switch has a circuit board e.g., for a WiFi switch like the WeMo, is the ground on the circuit board directly connected to the electrical ground in the gang box?
I'd guess that it is but if some knows for sure, please answer.
Then the only issue is, does any electrical standard frown upon sending e.g., 5VDC or 5VAC as I've described in the previous posts.

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