I'm pretty sure they won't make it available. They still don't allow uploading images to their service using an API, so I doubt they will allow to send messages

I'm pretty sure they won't make it available. They still don't allow uploading images to their service using an API, so I doubt they will allow to send messages. After the Twitter example, APIs are not attractive to these kind of startups. If you give full capabilities to 3rd party developers, they're going to create clients for other systems and the startup will lose control over its content and how it's shown to users (ads).