Problem solved, huh?
Sites are shut down daily for violating DMCA. Individuals' internet connections shut down, activity forwarded to authorities. Occasional prosecutions of the worst offenders, sometimes ending in suicide (why, just today, in fact). ITAR is the same sort of mechanism, but with national security implications. It won't be hack-job law firms for record companies pursuing charges, but none other than the State Department.
No, I don't think being forced onto the dark net or Tor connections to participate in a perfectly legal and moral hobby is something I'm okay with. In fact, that kind of totalitarian scenario is precisely why our government set up Tor in the first place. So people in China could learn about freedom without being arrested (maybe. If they're careful and lucky).
If you actually read the ITAR text, it is clear that an easily-circumvented approach like filtering will satisfy no one. This stuff will not be permitted to be accessed by foreign persons. That includes people in the US. And that means that the only way to host a site discussing the items is for a fully-encrypted, domestically-located intranet incapable of accessing forign servers, whose users must each be registered as US persons cleared to access ITAR data, using secure logons. No one will go to the trouble. No one. Even more troubling, the rules stipulate that anyone engaged in creating ITAR defense articles must be registered to do so with the State Department. So that registration of US persons would have to happen for anyone seeking to innovate in the hobby, whether they intend to publish online or not.
The best result will be that discussion of such things openly will result in a mod shutting down the thread, deleting offending posts, and punishing posters with bans or reprimands. That is what is done on most gun forums for people posting illegal firearms conversion details (or even discussing them), and that information itself is not even illegal. Worst case, forum moderators will feel compelled to not only shut down, ban, and delete the posts/posters, but also report the posters to authorities (because the post itself constitutes a crime under ITAR)
It might seem difficult or even impossible to enforce such a Draconian regulation, but look at the NFA or even the tax code, which are similarly regulations drafted by enforcement bodies. Look at the laws against espionage and how doggedly authorities have pursued Wiki-leakers and whistleblowers.
TCB