The only ways I know to do it are,
1- Go to your video manager, then community, then subscribers. From there, all of them have a message bubble in the right of there listing. If you click it, a message box will pop up for you to message them. The drawback is you have to do it one at a time.
2- Post a video with your message.
3- Try to get them to all follow you on twitter or other similar social media. (You can set your videos to auto post to twitter which is nice if you get them to follow you.)
4- Private message them with an invite to a blog or something to get updates kind of like a news letter.
The best would probably to post a video. They are subscribing to you to watch them anyway right? Even if the video is just a minute or so long, post it and let them know. Most people I have seen do all of their updates this way. But, if nothing else, advertise twitter, a news letter, or anything like that in your videos and urge them to join it.