If it's a bit of fun in Brockwell park for people already sympathetic to the "green" cause and interested in hearing people talk about a few other issues as well, then that's fine, although as far as the green message is concerned, it's mainly a case of preaching to the converted.
This has always been one of the biggest problems with certain sections of the Green scene, particularly the Brixton one. It seems to me the most successful Green thinkers / leaders are the ones who are most accessible to the wider audience. Being too closely associated with people
who appear to be a bit mad (and there are plenty of them in the Green movement) is likely to alienate people who would otherwise be persuaded by the science and the economics. It's hard enough to persuade people to forgoe their immediate self-interest without giving them a ready made sub-conscious excuse for not doing so.
I also think that serious questions have to be asked about whether Lambeth Council are alienated in a similar fashion. What language is used and what causes are advanced will impact heavily on the willingness of council officers and politicians to support or oppose. When I heard about the posters coming down, it seemed very fishy. Lambeth have not exactly been supportive in the past, so it would not surprise me if there was something underlying the posters coming down.
My experience of a few Green events, particularly the Big Green Gathering and others run by the same small group of Green event organisers is that they are very ghettoised, with the same people doing the same things year in year out As such it doesn't seem to move forward. Hippy / new age costume and rhetoric may sell well internally but it does down terribly externally, at least in certain circles (as we know only too well !). Thinking more about this issu of 'positioning' would help in the future.
I also think that the vision of a society in which we are all making our own beer, growing our own veg etc is not a very attractive one for many people, akin to the mud-huts, future primitive ideological bent of some. A more high-tech, specialised division of labour is both more likely and more saleable. There are people and thinkers out there, such as the New Economics Foundation, the Centre for Alternative Techology and so on, who are doing this thinking. A bigger prescence for them would help the event be taken more seriously.
As for the 9/11 conspiracies, the people who say that there are more important things to focus on are probably right. You don't need a 9/11 conspiracy to show that the neo-cons and neo-liberals are fucking it up. Resources spent promoting conspiracy theories of limited relevance are wasted resources, better spent elsewhere, unless that is the priority is entertaining a bunch of hippes in a field rather than acheiving meaningful social change.