Word is it's not possible. 08 Highlander comes with it, and apparently Prius comes with it outside the US (or NA or something), so it's actually built with the capability and people can add the EV button themselves. But what I hear is TCH has no capacity for having an EV button so there's really nothing to do.

And yeah I agree, before I even found out there was such a thing, I was wishing I had exactly that.