OKAY so it turns out that Arslan had already been adapted to manga (and an OVA!) once before:
Sooo yeah. A bit too hardcore shoujo for my taste, I'll probably stick to Arakawa's version xD
FMA's entry on TV Tropes' Better Than It Sounds page:
Military specialist searches for a magic rock recipe to get his brother out of a can. In one version of the story, the rock is their father, in the other version the rock is us.
Or: Children see a Thing Man Was Not Meant To Know, run around country looking for another one.