audiocableguy, I think you might have either a: not given the JL Tech all of the information necessary for him to ACCURATELY answer your question, or b: misunderstood what the question was in the first place, or c: a little of both. Just reading your post, I cannot tell what you posed to the tech as the actual question, or what his actual response to your query was.
The lack of motion is really the same as holding the cone, right? So, with all of the information already provided, it can be PROVEN, BEYOND A SHADOW OF A DOUBT, that high frequencies CANNOT damage a woofer. Period. Those facts are:
1: N0 (that's "Eta null") is the amount of power INPUT to a woofer that is actually converted to AUDIO. Typically .4% or so, and I have never seen higher than .7% myself. (Dan Wiggins mentioned at some time or another N0 of as high as 1%, but I've never seen it, personally) If we have, for example, a woofer with an N0 of .5%, and a power rating of 1000 watts, the math is applied like this 1000*.5%=5. 5 watts of input power FROM the amplifier is being converted to sound. The remaining 995 watts of that power is being dissipated as HEAT, IN THE VOICE COIL. Whether the coil is moving or not, that additional 5 watts, if not being "turned into sound" (because the sound isn't happening, right?) will NOT thermally stress the voice coil. It can't happen. Before you say "Yeah, but woofers move a lot, and that cools the voice coil", consider a 200 watt mid-range or a 100 watt tweeter... Those voice coils don't move very much at all, for "cooling". And the math still applies exactly the same.
2: Steven mentioned the automatic low-pass function of a woofer's voice coil, this causing the power available from the amplifier to become less and less. The inductance of the coil will simply limit the power applied, and as with all inductors, (a device the resists high-frequency CURRENT, the other half of the power formula - voltage will remain the same, but the inductance resists the current - without which, there is no POWER, or HEAT) the more power you apply, the more it will resist that power, as the resistance increases. Additionally, the higher the frequency, the more that inductance will try to resist that power even further. A 6dB low-pass filter. Simple as that. By the time you reach 1000Hz, from a reference of 500Hz, the POWER APPLIED will be 6dB lower, or 1/4 the power applied, even if the amplifier is still providing the SAME VOLTAGE. Notice that I am NOT saying anything about how much SOUND the woofer cone is making, I am referring
specifically to power levels, expressed in dB.
You simply CANNOT DAMAGE A WOOFER just by feeding it high frequencies, within it's power specification. If you overpower it, THEN you can obviously damage it, but that damage WILL BE FREQUENCY INDEPENDENT! It will be thermal damage. Period.
It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."