While an interesting thought, my company is in the uninterrupted power generation/supply business and its anything but easy to configure larger sites. I dont know specs of a Tesla but if the grid disappears we usually have a transfer switch somewhere in there between the grid, the load and the UPS to handle load transfer and I doubt the place where you plugin your car is adequate for this. Usually the various devices in the new microgrid now have to synchronize with each other and someone has to be the grid reference now that the actual grid is down and that can be a pain. If I have 2 Teslas and a gas powered generator all three have to agree who is master and who is slave and to actually be a UPS solution there better not be any drop in the wave form during that or its not gonna be a UPS for very long. I could see something like this more likely at a place of work where it can be controlled but I'm not sure I'd want my car supporting my office building in a power outage as it may drain it enough that I'm not getting home.
Anyway if I were Tesla I would not want the car that I build to now be liable for keeping power up for houses and businesses in an environment I've never seen and have no control over. There is no way they could support a warranty for that.