Actually, Barnabus figured out where the character's location was stored in the data.000 file; I just posted a translation and added a few coordinates.
That method would get you into the guilds without having to join, but might cause problems, since you would basically be skipping part of the game to get ahead of yourself. There are a couple cut-scene conversations in the assassin's guild, for example, which you may or may not be able to trigger out-of-order, which then may or may not cause problems with the related quest.