Edit the file /usr/share/xbmc/system/keymaps/keyboard.xml
and change:
<volume_mute>Mute</volume_mute>
to
<volume_mute>Screenshot</volume_mute>
then you can use mute button to grab a screenshot when using the XBMC plugin. If it opens a context menu then you know it works and you can then select where you want the screenshot files saved.
Thanks go to Shiro from BH team