The mention of this game is super nostalgic for me... yet the aspects of the experience I liked the least, are the only ones I'd expect to get captured properly in a manga.
'Twas a game that (according to memories from young me) dripped atmosphere and was chock full of cool stuff and even fun (if occasionally frustrating) gameplay... But the plot itself that I can remember was a blur of cliches and over-complications and and maybe a few good twists (caveat: I'm only going off the first half or so of the game which is as far as I got on my friend's PS1. I recall hearing of some cool bits, by child-me's standards, in the run up to the end... for whatever that amounts to).
That is, the story made for a wonderful grand adventure, when combined with the rest of the game, but was not to my recollection a good story as such. A manga-promo might have been self-defeating, especially without putting the game's awesome eye-catching "dragoon" armor on the cover (the art design of which, really made the game for me as a kid).