Update: I checking my Tesla account page, and it will allow me to puchase the Extended Maintenance Plan (I got as far as entering my CC info). Obviously, I could get an email from someone saying that it's not allowed, but it seems like it is... No similar option for the ESA. Again, I am inquring, as I don't see why it shouldn't. There was NO stipulation against it when I bought it.
If anyone would like to make offers conditional on me successfully purchasing the ESA and/or Maintenance Plan PRIOR to concluding the sale, I would be happy to entertain that.