The most important thing to remember when designing FLRPs, setting information, or even characters for the White City, is that you are playing in a Shared World. In any roleplaying game, you have an obligation to you fellow players to make the game as much fun (or, for you pretentious types, as powerful an experience) as possible. In a shared world, that burden of responsibility increases massively. Any element you add to a game fills a niche where somebody else could have added something.
Building a shared world is like building a wall. It is vitally important that everything that is built, is built on what has gone before. It is equally important that everything that is built can its self be built on. Everything that is added to the world must be consistent with what is in the world already, and it must create more potential than it consumes.
When writing for a shared world, you must bear in mind at all times the purpose for which you are writing. For a LARP, for example, setting information should either contain obvious adventure seeds, suggest new and unusual character concepts or otherwise expand the world, while keeping a consistent flavour.
You need to remember that your readers are also your peers. Whatever you may think, your ideas are probably no better than theirs. Bear this in mind in your writing, and make certain that at no point do you start to imply otherwise. It is always better to add something new than to redefine something old. That way you're saying "hey, look at this cool idea I just had" and not "hey, look at this cool idea I just had that's better than any ideas you lot could have had".
Remember that, however good a writer you think you are, most people don't LARP for the fiction. Keep anything you write short, flavourful and to the point. People will just resent having to wade through information that does not concern (a) them (b) their characters or (c) adventures they plan on running. Anything that isn't a plot hook or a character idea or a really, really interesting bit of flavour should be struck out with force.
Remember above all that this is a LARP group, and a LARP group of the kind that has Adventurers bodding about the world at random, fighting things for no clear reason. New content should improve the variety, quality and verisimillitude of those adventures. Details of political structures, quoracy rules and economics are all well and good, but if they don't give you a decent idea for a filler encounter in the Western Forests they don't do anybody the blindest bit of good.