While there is almost certainly a way to do it (youtube offers all sorts of editing afterall) I'm going to be as frank as I can and say that finding that sort of knowledge here is probably not going to happen.
What I would suggest is that you manually edit the video and then upload it for your users to download. Either that, or you find some video editing code (look in the snippets forum - you may get lucky) and then input each contestants line crossing time and let the code do its thing (though if you have to watch it to get each persons time you might as well edit it manually).
My helpful sig is on vacation trying to loose some weight. It got a bit fat and caused a few problems but it will be back at some point!