When building a voltage divider circuit, check how much current is going to be drawn by the “input” (i.e. the output side of the voltage divider). If you have a higher current input, you will need to use lower value resistors to get a stable voltage division. Also, please take into note the presence of any pull-up or pull-down resistors on the input. Again, these may also require you to use lower value resistors in your voltage divider, because otherwise, those pull-up or pull-down resistors are in effect resistors in parallel, which will significantly decrease the resistance of one of the legs of your voltage divider, rendering it inaccurate.
20200308/https://en.wikipedia.org/wiki/Voltage_divider#Loading_effect
Simple circuit ways to send a signal down a power line. High voltage communications and comparator. The other alternative of modulating the power supply at the same voltage is more challenging. For data send, design a local power supply that can store up all required charge to consume during blanking periods, and variably modulate the signal of consumption or high impedance (no consumption). Then listen for a modulated power supply signal for data receive, again using your local energy storage to continue operating when your power supply goes blank.
You may have to do that because of this problem… if you just have a carrier-wave at the same voltage as the power, can you really filter that and tune to it with just capacitors? It’s going to be a low amplitude signal compared to your power supply, so after tuning with a capacitor, you might need to amplify the signal quite a bit.