@edmcnierney I saved a Pixel C just for today!
I kept a PIxel C in the box unopened, so that I could do a last minute confirmation that my .img files work, without being affected by any modifications.
I want to share the exact procedure I used to flash the device, starting from an unopened Pixel C box shipped by the XPRIZE foundation. If the testing lab uses the same procedure as I did, everything should be fine.
(1) Charge the new Pixel C
(2) boot up. Connect to an internat wifi (the setup wizard won't continue without you doing this). Skip all google account info etc.
(3) The device will be forceably upgraded to Android 7.1.1. Let it complete.
(4) Boot again into the setup wizard. Skip setting up accounts and sharing info with Google where possible.
(5) Once in the launcher (of Android 7.1.1), go to System settings. Tap on the build number 7 times to become a developer.
(6) In developer options, allow OEM unlock.
(7) Turn the tablet off
(8) Turn the tablet on holding down the power and volume down keys. Enter fastboot mode.
(9) download the factory MXC89L from the Google factory images site here:
(10) unpack the zip.
(11) run the flash_all script inside the zip archive
(12) reboot back into the bootloader
fastboot flash boot.img
fastboot flash system.img
fastboot flash userdata.img
(14) The system should reboot into my customized system.
This may sound complicated, but as far as I am aware, this is the simplest and the officially supported way to downgrade a Nexus tablet. Since I tested this on a completely untouched tablet, I can confirm that if the testing lab sets up the tablets this way, the images will boot and run.