Easiest way: Play the Hastings battle a few times, maybe a dozen times. Instead of showing a movie of the whole battle, simply edit together close-ups of certain events from the multiple battles and tell a story out of it. You can make the Saxons playable in the Hastings desc_battle file the same way you unlock factions in the campaign. Play as both sides to get them to do what you want, it won't matter what the whole battle looks like at the moment, all you have to do is show a close up of them them charging, holding spearwall, getting charged by knights, shooting arrows, getting hit by arrows, etc. That way you can use footage from any battle it happened in.

Terribly difficult way:
Perhaps with a little modding (of which I know next to nothing), you could make the SP Hastings battle into MP. Use the MP 100 Years War descr_battle as a template and replace the map and army information with those from the Hastings Battle.

Just a few ideas, I have never tried anything remotely close to this, but there must be a way. The History Channel had a series a few years ago where they recreated famous battles in RTW, which is the same.