Oh, dear, I'm exceptionally stupid. What I meant to say is:
Stative = permanent state (ie. "Bob is silly", implying that Bob is always, permanently silly, and can always be considered as such)
Essive = Temporary state (ie. "Bob is being silly", implying that, while Bob is silly at the moment, he may (and in this case, probably will) stop being silly at some point, and be serious.
Waltke and O'Connor make this distinction (section 22.3, p. 367ff), but upon looking this up, I note that they simply refer to them as two kinds of statives. If there's a more common word for "essive", I'm happy to hear it. For those who don't have it, http://books.google.com.au/books?id=jZl ... ve&f=false
may be helpful (or may not).
Wiktionary acknowledges my use of the word as "rare", and confirms your guess as to the origin being "esse". https://en.wiktionary.org/wiki/essive
I also ran across an article with "Greek" and "essive" in the title, but it seems to be mainly about Indo-European, and didn't seem to explain what Essives were. http://lingu.glossa.dk/essive.html
So (assuming I've managed to make myself clear this time), is there another word I should be using?