[development] hook_user_operations permissions

I'm implementing a bulk user operation as described in sirkitree's blog post: http://sirkitree.net/node/23 I don't see anything in the hook_node_operations() docs that addresses permissions. I'd like to provide discrete control over whether my operation is available, rather than having it show up in every VBO view that happens to return users. Do I do that by having my implementation of hook_user_operations return a null array if the current user has no permissions? Or should I conditionally define my implementation of hook_user_operations depending on a user_access() call? Or am I missing some other option?