Ubuntu iMessage Client
This message is quoted from https://apple.stackexchange.com/a/66047:
iMessage is based on a propietary, binary and encrypted protocol (source Wikipedia) with a client side device certificate, which makes it unlikely to be reverse engineered and implemented in third party apps any time soon (although attempts are being made).
Edit
It may be possible in the future to run the native OS X applications on a Linux OS similar to how we can run Windows applications using wine's compatibility layers. The Darling Project aims to do that. However, I have never encountered a situation where I needed to run a native mac application, so I have never tested it. This does seem promising, though somewhat doubtful. The following is quoted from wikipedia:
Just like APNS it sets up a Keep-Alive connection with the Apple servers. Every connection has its own unique code, which acts as an identifier for the route that should be used to send a message to a specific device. The connection is encrypted with TLS using a client side certificate, that is requested by the device on the activation of iMessage.
Well you can't directly, but there are roundabout ways.
Here are two:
Both require an iDevice though. So no, you cant do what you wish, but you can send info from your PC, and have it delivered to your iDevice, and then it sends it. both methods require a jailbroken iDevice.
SSH. You can SSH into your iDevice with putty or something, then send the messages via terminal.
Remote messages: Just install the cydia app, then open your web browser, and type in your idevices ip address, followed by a port number (example:
http://192.168.1.3:333/). This will work on any platform, even your phone. You can take it a step further by enabling port forwarding, so you can access a device halfway around the world.
Others:
You can always send text messages. Try using curl. Here's a line (If I remember it correctly)
curl http://textbelt.com/text -d number=9876543210 -d "message=put your message here"
There are other messaging tools available as well. If your a hacker/prankster, you can try SET (social engineering tool).
I'm sorry for not being able to supply you with a solution, only alternatives.