I've been watching this thread for awhile, but the new forum migration made it so I couldn't sign in to respond. But it's all working now, so I'm back again.
Very early on in development I sketched out a number of ways that players could be involved with the Battle of the Hornburg in such a way that they would never have shown up in the pages of the book. It was very elaborate, and more clever than good, if we're being honest. It involved a lot of dressing up in armour and wearing helmets to mask your features and leaving through side doors just when Aragorn and company arrived, and showing up immediately after something cool happened and all of the main characters had already exited stage right, like in a 'Lend Me A Tenor' or 'Rumors' style farce. Hobbits would have been mistaken for children a lot, and told to wait in the caves.
It would have all worked, and would have adhered very strictly to the words of the book, but its biggest problem (aside from providing a rather questionable degree of that rare commodity 'fun') was that it just wasn't believable at all. It really strained the Credibility Wall. Why would players actively avoid running into the main characters? Why inject some grand conspiracy where none exists? It felt like such an approach, while technically correct when comparing it to the source material, was just a Bad Idea for the health of the story. This is supposed to be a singularly important battle for the survival of Rohan, not a 'Where's Waldo?' game of finding out where we can sneak players in between the gaps in the pages.
It still feels legitimate, actually. It feels like the Battle of the Hornburg. I'm glad your characters are going to get to see it in a way that lets it feel like the Battle of the Hornburg, without having to hide behind an NPC instead, for fear that Legolas might spot you.