DC Power Line Communication (PLC) low pass filters
I would expect the PLC devices to connect to the power line through a diplexer. The data signal would be connected to the power line through a high pass filter (not a low pass filter). The PLC device might use a low pass filter to connect its power input to the power line, but it's possible this isn't needed.

You'd normally want to design your PLC system so that simple loads don't need to be modified when PLC devices are added to the power bus. If this is done right, then your DEVICE1 and DEVICE2 (which use the bus only for power) wouldn't need any special filter. If it isn't done right, or if you can't control how some of the power-only devices are designed, you might need to add a low pass filter to the connection for the power-only devices.