There's a lot to like about Sony's latest generation of Android devices. One od the things that most people don't like is the custom interface that Sony puts on pretty much everything. If you want to do away with it and get some sweet, clean Android Open Source Project code running on your shiny new Xperia Tablet Z, Sony is happy to oblige. They've posted an AOSP 4.2 build for the Tablet Z to GitHub, following their surprisingly open approach to other devices, most recently the Xperia Z flagship.
Sorry ROM aficionados, there's no flashable ROM package posted. Developers will need both the AOSP code and the binaries for their specific device (which you can find here) to build a working ROM, and end users need an unlocked bootloader, with the voided warranty that comes with it.