Avoiding haecceities

Continuing the discussion of the 'thin' conception of existence Bill Vallicella directs us to a post from last year's round:  C. J. F. Williams' Analysis of 'I Might Not Have Existed'.   Bill quotes from Williams 

. . . my assertion that I might not have existed is the assertion 
that there is some property . . . essential to me, which I alone 
possess, and which might never have been uniquely instantiated . .
     (What is Existence?, Oxford 1981, p. 104)

Williams appears to be advocating an haecceity property, anathema to the Maverick.  But do we really need anything this strong?  Can we not say that a sufficient condition for my not existing is that some property I do instantiate be uninstantiated?  In other words, rather than asking for some sort of conjunction over properties that captures me exactly, we define a condition on possible worlds by quantifying over properties.  Could this be a necessary condition too?  After all, if I don't exist in world w then surely we can find some property that I possess uniquely in this world that is not instantiated in w, if you see what I mean.  But then again I am horribly modally-challenged, according to BV, and I think he's right.  Maybe I can talk the modal talk but I can't walk the modal walk, that's for sure.