Ayami Kojima's character designs CAN be rendered in 3D quite easily(maybe more easily than Amano's). Anybody who says otherwise lacks vision. It's not like Kojima's going to design backgrounds too. There's different artists that work on games, and some are specifically dedicated to character designs(which Ayami would be), while others are dedicated to the world's look. Just like in the Final Fantasy games, Tetsuya Nomura DOESN'T design the look of the world, only the characters. You have other artists like Yusuke Naora and Isamu Kamikokuryo who particularly create the way the world looks(as well as art pieces depicting Nomura's characters WITHIN the world, which interestingly enough, often are credited to Nomura falsely). Just like making a movie, creating a video game is a team effort. It's not the old NES days where you had only a couple of artists working on the game's look. Each artist adds a bit of their own flair to the project.
But yeah, I believe Ayami's character designs CAN be translated fairly damn well if they were to make a new CV game for, say, PS3 or X360, and asked her to participate. The probablity of their rendering coming off good is probably the same as Nomura's. Amano would harder, considering his style is more watercolor and less sharp(unless he's doing a pure ink art piece). Though, as seen in Dissidia, you CAN have a "middle man" who takes character designs that are hard to translate into 3D models(in Dissida's case, Nomura) and MAKE then easier while still retaining the basic look and feel of the original designs. I've always been a fan of Amano's work, and it killed me that we might never see it in a video game looking true to his style(his style is eccentric, motley, airy with a mixture of Eastern and Western cultural styles). But with certain artists acting as "middle men", his work CAN be idealized in video games(even more so now than ever before).