Though we plan to have a 80% "cut off" (some trench milled into the PCB to allow kbd to bend without making the BGA chips on other half of PCB go 'pop'), and thus a "breaking off" of kbd PCB half was theoretically possible maybe, you still would need to place that other half of upper-PCB - with its display connector and whatnot - somewhere, so nothing gained by such design regarding thickness of device. Sorry./j
You rather should think about placing additional battery volume underneath the "roof" formed by slide-up display. There's available volume of roughly 23*12*110 mm^3 which makes for quite some LiIon-stored energy. If you want to lift the kbd a few mm to make it flush and plane with display, you easily can attach a second kbd to replace the original one. /j