Second solution. This time based on bluetooth.

Download [Bluetooth Proximity Tasker](https://itunes.apple.com/us/app/bluetooth-proximity-tasker/id525925062). Sorry mate, but it's $2.99.

> Run any AppleScript when a paired bluetooth device goes in and out of range. Adjust the range using signal strength.

Select your BT device and paste this script on `In Range Applescript`:

    tell application "System Events"
        tell process "Messages"
            tell menu bar 1
                tell menu bar item "Messages"
                    tell menu "Messages"
                        tell menu item "My Status"
                            tell menu "My Status"
                                    click menu item "Available"
                            end tell
                        end tell
                    end tell
                 end tell
            end tell
        end tell
    end tell

Replace `Available` with `Away` in the script and paste on `Out Of Range Applescript`.