Well the EMP effect is apparently only 1 km in size. So that should simplify the process.
Also,
If they were radio waves, It would have been stated in one of the several references, “they’re radio waves” instead of using language like “propagate in a similar manner to radio waves.”
“Appropriate equipment,” not oh, any ole receiver on hand. Several mentions in the sources of “receivers,” not recipients or targets or destinations.
And the only canon mention I can find it interception was by (pre-)ComStar itself.
Strategic Operations Pg.251: "While the “jump” involved
in sending the signal over many light-years carries with it all
the usual hyperspace issues, like I was talking about with emergence
wave detectors earlier, the actual signal is a conventional
electromagnetic signal, generally a radio frequency burst.
Think about that a second: you don’t need an HPG to receive
an HPG message. You need a radio."
&
"While the radio signals from an arriving HPG burst can
propagate up to 4AU from the arrival point..."
Amongst other things I am now somewhat concerned about the level of funding for intelligence operations in the Inner Sphere, Pg.210: "...attempting to tap into interstellar communications. Fortunately for the Great Houses, this problem is not insurmountable.
The Kearny-Fuchida principles at the core of HPG technology
involve a degree of electromagnetic propagation that
sufficiently
well equipped agencies can listen to, though this is still extremely
limited;"