SDesk is just calling out to a script to play the video (apps/playmovie for single playing and apps/playvideoqueue). That script will call mplay. I briefly looked at the new DefaultZoom option, and as far as I can tell, that is only used as a default value when browsing via the mplay interface. I didn't see an explicit command line flag to set that.
My suggestion would be to add some code the the video start script in sdesk to produce a default .mplay file. Here is some sample code... I don't know what the correct option names and values are (I couldn't find any docs on MPlay that contained a list of .mplay options of folder.cgf options), but something along this line might work.
# Create a .mplay file if none exists...
FILENAME=`basename "$2" .mpg`
if [ -f $MPLAYNAME ]; then
echo dontcreate >> /tmp/playmovie.log
echo create >> /tmp/playmovie.log
echo "DefaultZoom=3" > $MPLAYNAME
I probably won't have time to look into this, as I am winding down my work on SDesk.