You can get a KNS80 FM Immune (with antenna lead filters) and you can even get the installation BRNAV certified. I know a man (this sounds like the yellow pages advert) who has got one of these.
Unfortunately it is close to useless, because ATC routinely give you DCT legs way outside the DOC of the navaid defining the waypoint. I get such a DCT on perhaps 50% of IFR flights. And the rest of the time I ask for one, to get a shortcut
On Saturday I got a DCT across much of France, to some place in the Channel (very nice).
Obviously there is a way to hack such a situation with a KNS80, tuned into a series of
nearby navaids, one after the other, but it escapes me how you could do it in a hurry. You would have to assume that you will fly the
filed route in entirety (never the case) and you would have to pre-plan all the RNAV waypoints in between. This has to be done right because if ATC gives you a DCT XXX where XXX is 150nm away, and when they see (say 30 secs later) that your track to XXX is 10 degrees off, they will be on top of you. This is bad enough when I am 10 degrees off at the start of a leg because the autopilot is slow to adjust the wind offset...
That's why I think a KNS80 is a good doorstop and little else.