WITH SOME CLARIFICATIONS HELPED ALONG BY THE ENGLISH RELEASE, the KH cast has decided that Ventus is appable, but I would still request that you speak to me about it.
Roxas and Ventus share a heart! And as a result, canon tells me that Roxas responds kind of badly when he's around his heart, as in, falls asleep, and so forth. Because Ventus is one half of his real self. Sora is obviously the other half. We would need to work with that, as it would be a matter of coming up with how Ventus could be awake in the heart sharing, or if camp allows them to both co-exist in camp, and all the obvious angsty implications of that.
But! They have individual bodies, and histories and so forth, but the heart matter is kind of a huge deal.
(There is also the possible implication that Roxas is what happens when you put together Vanitas and Ventus. That's something we might wanna discuss, too.)