I photographed it 5 years ago -- there were a lot more photographers than zombies, although if they're up to 6000 zombies then I think the tide has turned (or maybe there a lot more photographers-turned-zombies about?). It's a lot of fun if you have a chance to go, especially capturing the expressions of onlookers who are not quite sure how they ended up in the path of a zombie procession.
I was there last year and there was no shortage of zombies! Definitely a great event with lots of photographic opportunities. If I go again this year I may bring my TLR along...
I thought about a Mamiya 645 with a power grip or a Bronica SQ-B but I may succumb to dragging the "monster" RB67 Pro S to the party. Friends who shot MF the last few marches had no problems arranging posed zombies. Should be fun.That's why I posted.