Is there a RF signal device for a 120 or 240 vac wire to start and stop a standby generator?
Transmitters can be capacitively coupled to a power transmission line, but this may not even be necessary in your case.  In the short distances you are stating, a completely wireless solution should suffice, given there is a power source at the remote generator.

However, it seems to me in my initial thoughts, that a load sensing circuit could be installed at the generator end that, along with a relay, could serve your purposes without having to install some over-complicated wireless system.  That, however, would require a separate question, like "How can I engage a remote generator using only the supply wires that are currently installed"...