Here is an example of St Severus' letters:
"Enough has, I think, been said about essence and hypostasis. But the name 'nature' is sometimes taken in place of essence', sometimes in place of hypostasis. For even the whole of mankind we call comprehensively 'nature', as it is indeed written: -Ãƒâ€šÃ‚Â½For all natures of beasts and of birds, and of reptiles and of things that are in the water are subjected and are made subject to human nature-+: and again we speak of one nature in reference to a single man, Paul for example or Peter, or maybe James. Where therefore we name all mankind one nature, we use the name 'nature' generically in place of 'essence' 63; but, where we say that there is one nature of Paul, the name 'nature' is employed in place of 'individual hypostasis'. So also we call the Holy Trinity one nature, employing the term 'nature' in place of the general designation 'essence'; as Gregory the Theologian the bishop of Nazianzus also said in the sermon on the Holy Pentecost: -Ãƒâ€šÃ‚Â½Confess the Trinity to be of one Godhead, my friends; or, if you like, of one nature; and we will ask for you from the Spirit the expression 'God'-+. But, when we say 'one incarnate nature of God the Word', as Athanasius the prop of the truth and the apostolic faith said in the books on the Incarnation of the Word , we use 'nature' in place of 'individual designation', denoting the one hypostasis of the Word himself, like that of Peter also or of Paul, or of any other single man. Wherefore also, when we say 'one nature which became incarnate', we do not say it absolutely, but by adding 'one nature of the Word himself clearly denote the one hypostasis. But the very men who blasphemously call the one Christ two natures use the name 'nature' in place of 'individual designation', saying that the Word of God is one nature, and the man as they say from Mary another. For they do not reach such a height of fatuity as to say that they are using the name 'natures' in place of 'general designation', I mean in the same sense as essence: for, if the Holy Trinity is one nature, and all mankind one nature, in the same sense as anything which is shown to be so on this principle, the Holy Trinity will be found (to say a very absurd thing) to have become incarnate in all mankind, that is the human race."
It is clear from this that St Severus used nature, in the phrase 'one incarnate nature of the Word', to mean hypostasis and individual designation. He is well aware of the distinction between essence/generality and hypostasis/particularity. Indeed you can see here the criticism he levels against the Chalcedonians of his time -> if by two natures you mean 'two individual designations' then you are Nestorians, and if you mean 'two generalities' then you are saying that the whole Trinity has been incarnate. This at least shows that he considered that there was a justifiable criticism of Chalcedonianism to be made.
I will stop rambling. Christology, and the efforts for unity between Orthodox are my most consuming interest.
Have you seen my site www.orthodoxunity.org?