This post from Chuck--no longer on staff--claimed they'd publish the whitelist, but that was almost two years ago (shortly after they either implemented the whitelist or made it more restrictive; I can't remember how it was before this). Obviously, that hasn't happened, so I probably wouldn't count on it any time soon. But it looks like they were open to it at one point:
But his advice still stands: if there's something you want to do and you're used to Java, there might be a way to do in Groovy without the import you are used to using. Staff also are or at least were open to reasonable feedback on this list (though given that it's been two years and I don't recall any major changes recently, it might be safe to say that they consider it reasonable at this point).
In the meantime, I guess all you can do is try and see if it works. 
If not, the community (and staff) may have some ideas if you have a specific goal you don't see a way to achieve as-is.
