Use environment variable in guisettings.xml

Ronie,

I would like to change entries in guisettings.xml like:

<setting type="string" name="skin.transparency.Custom_Home_Movies_Fanart_Background_Path">/Users/mwinestock/Library/Application Support/XBMC/userdata/Thumbnails/Video/Fanart/</setting>

To:

<setting type="string" name="skin.transparency.Custom_Home_Movies_Fanart_Background_Path">$HOME/Library/Application Support/XBMC/userdata/Thumbnails/Video/Fanart/</setting>

How do I go about using a environment variable in guisettings.xml.