I don't mind posting the idea because (1) I don't have or anticipate anytime soon getting an extra car to put it on, and (2) it's been unavailable as long as I've been checking, perhaps even still registered to the original vehicle?
I currently have three Teslas, each with custom plates:
1) 2008 Roadster, 'BB VIII' (BB-8, the Star Wars droid)
2) 2012 Model S, 'RCHRGBL' ("Rechargeable", though sometimes people ask if it means "Rich Gerbil"?)
3) 2017 Model 3, 'OHM NOMM'.
I did originally want to get 'BATT MBL' for the Roadster, but it was taken, alas.