Plan is to not touch screen housing + slider at all (it's already made of metal), which both make design much less complicated and less material is needed. While I appreciate ideas and I'm grateful for them, transforming N900 into not-slided thing is a no-go for me.
On latest news side - I've measured possibilities of having socket for two 18650 LiIons instead of flat batteries, and it would require only few mm more thickness than mugen cover, giving 6000 mAh of real capacity, in return. Probably not an option for bigger backcover only, but something to consider for full body replacement. Do You imagine runtime on 6000 mAh? /Estel