Are you sure that the manga came first? When I read it, a lot of anime-only characters get 'cameos', despite not really have huge roles. A few HOLY officers and even Kimishima (and the Dars too, from the anime have one panel).
Either that's very poor writing to set up characters with literally a page worth of story or it's fan service. To me, it sounds more like the latter, and I know I've heard the anime came first. I'll have to look it up.
You really should watch those; they were amazing. Even though Kazuma and Ryuho are very different characters in the manga, both Hoshi and Midorikawa still beautifully bring them to life.
I couldn't read it anymore without hearing both of their voices.
Let me try to find them online for you if you haven't seen them. Sadly though, the DVDs only have a handful. Given the fact they put for all that effort (even casting for new characters), I can't imagine they just picked 5 chapters in Japan. It was probably complete, just the DVDs only included a couple.
As for AMVs, yeah, a couple by me:
This one is like my magnum opus AMV; easily my favorite I ever did. This is the culmination of all my love of S-CRY-ed and why I like editing it.
Just like the series itself, it's heavily Kazuma AND Ryuho based, and how they are opposite sides of the same coin.
Set towards the end, both of them get their moments and then the story finishes and continues.
https://www.youtube.com/watch?v=LM7jP7UPoO0
I've also done a purely Kazuma one, Ryuho one and way more than is ever necessary. But if you have to watch one; this one.