You know, it’s funny. In what is now 33+ years of driving, I’ve had exactly two citations for speeding. The first happened when I was young and dumb, and on my way to see my fiancee 7 hours away; the second happened when I was being an idiot driving on US 50 at the Utah/Nevada state line (by being an idiot, I mean driving 80+ mph in an attempt to make up time on my way from Denver to Monterey). As a rule, I drive within the speed limit, or, on rare occasions, not more than 5 mph over it. And I don’t have to worry about traps, or tickets, or being pulled over, or any of that stuff. All the bitching in the world about what state highway patrols do, or most (not all, yes, I know some small towns supplement revenue with true speed traps) local authorities do in the way of catching speeders would go away if people would, you know, stop speeding. 
Having said that, as far as the issue of flashing lights goes:
If you are flashing brights, you are in violation of the law probably in every state in the union. If you are turning your lights on and off, and it is dark enough that you are required to have them on, you are violating the law. If you need not have your lights on, and you flash them on in order to warn someone of a radar location, then some careful attention would have to be paid to what exactly you were charged with; creative attempts to extend the reach of laws not designed to be applied in such circumstance should be fought with vigor. If the local constabulary (or the state police force) are systematically using such efforts to try and stop someone from signalling the placement of a radar gun, then that certainly is a violation of your Fourteenth Amendment rights (due process being construed to include the right to free speech, incorporating the concept of the First Amendment), and should be stopped.
The potential difficulty is this: if you did indeed commit an illegal act, then you won’t be protected by the concept of the right to free speech. So if the law is being applied correctly (whatever law is being applied), you don’t get to say, “But Your Honor, I was breaking the law to communicate something, so I’m protected!” If the law is not being applied correctly, then the citation will be dismissed, without need to reference the issue of free speech (though you certainly might convince a judge not to apply a given law in the specific way the prosecution is asking if you indicate it’s being applied that way to chill free speech). Either way, you are not going to impact the efforts of the police force using that law that way. To accomplish that, you will have to file some sort of systemic lawsuit alleging violation of due process by the activities of the department in question, and that’s a much more substantial undertaking.
And of course, either way your ticket works out, you are spending your time in court. So there is a price you pay for exercising your right to speak freely.