[Leopold]

alarm_status is the part you are interested in.

0 = motion detection disabled
1 = motion detection enabled but not detected
2 = motion detected

Do you have an idea how to (easily) implement it?

It could probably be done but it would take some time to make the addon work completely with the MJPEG cameras, and my free time is quite limited at the moment.

Or do you at least see a way to have a button assigned to my remote control that brings up the cam as a picture-in-picture-screen,

For that you could use the "Security Cam Overlay" addon http://forum.xbmc.org/showthread.php?tid=182540.

You can run the addon with a button press by putting something like this in userdata/keymaps/remote.xml

Code:

<keymap>
  <global>
    <remote>
      <blue>XBMC.RunAddon(script.securitycam)</blue>
    </remote>
  </global>
</keymap>

The overlay grabs the focus so while it is active you can't navigate any other windows.