I picked up a surplus Radio Shack 43-218A wireless intercom. A search on
the
site says they are either on 200KHz, 230KHz or 270KHz depends on the
switch
setting A,B, or C.

I am considering modifing them so that I can put the HF rig audio into one
of
the them and be able to listen to the rig while in other areas of the
house.
Has anyone done this? Comments?

I would also like to understand how the 200KHz is inserted on to the power
line, does anyone have a schematic of these or similar units?
===================
I have such a intercom set as well. Upon transmission the built-in
loudspeaker of the 'that time' transmitter serves as a microphone.
Suggest you connect a small 1:1 or about audio transformer across that
loudspeaker . The other transformer winding can then be connected to the
output of the the HF receiver , perhaps with a 10 Ohms resistor across it.
The transformer provides safe galvanic separation .
In addition to the fixed resistor you can fit a parallel say 20 -50 Ohm
potmeter to adjust input signal to the transformer and hence to the
loudspeaker type microphone of the intercom -transmitter. During that type
of duty the intercom microphone might also act as a low level output
loudspeaker of the HF receiver output signal.
If accidentally the intercom 'transmitter ' is set to receive , the HF
receiver audio output stage won't get damaged.

The audio transformer is not critical . I can imagine you can even use a
small power transformer with 2 separate low voltage windings (it being not a
Hi-fi system). It involves some experimentation ........that's part of the
hobby !

If you don't like the above ,you can always try acoustic coupling between
the 'microphone' and a separate rx loudspeaker. Another experiment !
Amateur Radio can be a lively hands-on hobby.

Frank GM0CSZ / KN6WH