24 inch iMac Early 2009 - Use as monitor via Firewire

I'm trying to figure out how to use my old iMac as an external monitor for a windows laptop. I've read that it's impossible since its Mini DisplayPort doesn't accept input, but it seems like there's a possibility that nobody has considered. Firewire 800 seems to be capable of carrying video signals, and the machine has one of these ports. Is it possible to turn this old machine into a monitor using this port, or is there a good reason why no one has suggested this?