Great question, Nick.

I think that Winnicott would say that the transitional space isn't quite limited by the symbolic. Although at this point we're working with slightly different vocabularies, as Winnicott never talked of the 'symbolic' (though he did speak of frustration and the maternal phallus). He was much more interested, as you're putting in, in liminality--in transitions and movement, rather than abrupt introductions into something, e.g., the symbolic, into selfhood, etc.

Here I think Lacan and Winnicott are close, as they both escape rigid structuralism (for Lacan, it is the real and objet a; for Winnicott, it is transitional spaces). However, they both theorize subjectivity differently, and it has implications on clinical praxis.

But to your point about not being familiar with Winnicott: I know! He is not talked about outside of strictly clinical circles! I think he has a lot to offer about thinking and talking about human subjectivity, though, and I intend to bring that more to light in the coming weeks.