Sadly the Rasberry Pi has nearly no powermanagement and draws nearly 0.5W even in idle.
For comparison N900 draws 0.035W in idle.

Maybe an edison in the battery slot instead of the leather and a bigger Battery in the mugen cover.

https://talk.maemo.org/showpost.php?...9&postcount=25

enables x86 Software (like a Browser) too and better powermanagement (hopefully).