What would be a good idea is to post JW-critical events (apostafests, silent lambs ads, did-you-know-he-was-a-perv postings (if legal in your area/state)) prominently in the big 3 day convention centers. They are usually held in public centers and either have public posting areas or management may allow them to be posted. If you have either implied or expressed permission, the org. can't do ANYTHING about it and they cannot be removed.
If attendants out of spite do remove them (post early, check often), complain with management and/or the JW PR desk and/or the local constables and they might even be forced to announce your posters especially if you have written permission. Last year we had people posting something for what looked like a quiet fun 'worldly event' during the convention and JW security wasn't allowed to interfere with the raggedy dude posting them, some young witnesses reportedly 'checked it out' during the lunch break.
If you have the money, you could even rent out some ad space inside some of these centers for the month(s) the site is being used by JW - those ads are usually not allowed to be covered up. Imagine a big 'warning' message behind the speaker for all conventions held there that year.