Used until now this routine (giving 17MB free).

Code:
init 4
mv /usr/lib/enigma2/python/Plugins/Extensions /media/usb
mv /usr/lib/python2.7/site-packages /media/usb
ln -sfn /media/usb/Extensions /usr/lib/enigma2/python/Plugins/Extensions
ln -sfn /media/usb/site-packages /usr/lib/python2.7/site-packages 
init 3


This one give more (21MB) moving more. Nice.
Tested with OpenPLi 4.0 on DM800HD SE.
Thanks