Time to break out that old Nexus 7 or Nexus 10—Google has just posted system images based on Android 5.0.2 for both of these devices (razor and mantaray). The most recent version on these tablets before today was 5.0.1, so it's not like they've been waiting on Lollipop this whole time like some devices (cough razorg cough).
You can use the factory images to flash your device back to stock no matter what you've done to it. As long as you can access fastboot, you can flash the system image. There are also updated binaries with all the drivers needed to run ROMs on these devices. That's more important for developers, but if you tinker at all, the factory images will come in handy.
Keep in mind you need to have an unlocked bootloader to flash system images, which you do via ADB on a computer. There will probably be OTAs at some point in the not too distant future, so you don't have to mess with the factory images if you don't want to.