I do something like this, but I use the "New User Welcome" feature that was added to XenForo (I used to use the "New User Conversation" add-on for this but it's no longer necessary). Basically you've got an option to send your "welcome aboard" post to users via conversation, which may be preferable because it avoids posting duplicate content on the forum. If you want to do it that way, it's under Home > Options > User Registration in your ACP.