I ran UT on grouper way back in the day. It performed acceptably, until the switch was made away from surfaceflinger for the display manager. Then it became really crashy. Not long after that it was abandoned as a target platform. Based on that I imagine that even if you could find any old images, the device would not be very useful running that. For one, you wouldn't be able to find any other software to install on it. Lots of applications in the OpenStore would not even run on that version of the OS since it would contain old, or even have missing libraries.

I own one also but since I also have own newer Nexus devices I will focus on using them. Given the age of the 2012 devices I can't imaging much reason to open any new development and any system images that you do find will without doubt be full of bugs.