I'm sorry but restricting writers to just a few sentences seems a bit unfair to me. All a writer brings to a collab like this is his creativity, and you are preventing us from showing that in favour of someone else's creativity. As far as I'm concerned I don't feel I deserve to be credited on an animation if all I have contributed is a couple of lines of rubbish, which have then been interpreted and changed anyway.
If you want writers to take part, let writers take part. There is nothing wrong with a long submission so long as the actual plot is simple. My story for example was easy to summarise in your three sentence limit, but was originally 1000 words because I wanted to write something that was enjoyable to read without needing animators to make it worthwhile. Most of the story was visual description to inspire an animator and help to get across what I had in my mind when I was writing it- I wasn't expecting it to be literally stuck to, but interpreted with some ideas used and some scrapped. It was also written with music in mind, with rapid swings in emotion a perfect vehicle for a musical composition.
If every film maker made a film based word for word on a book or story then every film would be a week long. The fact is a writer has to include a mountain of description to make a story worthwhile, while an animation need not bother as it visualises the story for you. For me it is much easier to interpret and create something from a story which has description allowing me to visualise the plot, than it is to create something based off 3 lines that contain no description whatsoever and rely on myself being able to create all of the visualistion and ideas myself.
Two better solutions have already been made here- don't force animators to stick word for word to the story, but let them interpret the story however they want (personally I thought this was the case all along, and if it wasn't it certainly should be). If they want they can animate just a short section of it, or change whole sections so long as it is still loosely based on the original. This would free writers up to actually write something of some worth for this competition.
Alternatively writers need to be able to contribute their ideas to the animators once they have been chosen, essentially describing to them everything that the short sentence limit has forced us to excise. Pointless when we could just write it in the first place, but at least it would allow writers to get more involved and feel like a worthy part of the team.
I recognise that you are an animator yourself Tom, and probably favour animation over writing as after all that is what this site is founded on. I also realise that I have no idea how hard it is to create something in flash, and that overly complicated stories can be impossible to create in flash in such a tight time limit, but I feel that writers need to be allowed their own creative freedom if this is going to work.