According to KH Insider, Luxord and Larxene's VAs have been confirmed for 358/2. Looks like most of the VAs have been confirmed, but not Demyx's(Ryan O'Donohue).
IMDB has a list of 358/2 VA actors, but it's not always reliable and even though I trust it for other things, when it comes to a game that's not out yet, I trust IMDB about as far as I can throw it.
There's no way that I know of to contact Ryan O'Donohue either because if there were, you can bet that the more rabid Demyx fans would have gotten to him first. I guess it's wait and see as 358/2 is closer and closer to being released.
At least most of the others have been confirmed, especially Axel. I can't imagine anyone other than Quinton Flynn playing him.