I've oscillated a bit on this question for years. I went from being the most vulgar kind of historical materialist to an idealist and back again. Now I'd like to become a more sophisticated kind of materialist, one that acknowledges the possibility of things like hope and sociopolitical change. A large basis for this view is the Lacanian model of subjectivity. In contrast to the brute determinism of my historicist stage (in which the subject is always directly reducible to the sum influence of juridical forces acting upon it) and the solipsism of my drift into idealism, Lacan allows for a kind of radical freedom to emerge due to the way that subjectivity is premised on a constitutive gap. The subject isn't the direct result of external forces, but is the result of the failure of those forces. Copjec gives one of my favorite explanations of this.
This isn't just Lacan. It's also evident in Kant;s argument about the subject's stance towards the thing in itself. Searle expands on this position in Rationality in Action:
Kant pointed this out a long time ago: There is no way to think away your own freedom in the process of voluntary action because the process of deliberation itself can only proceed on the presupposition of freedom, on the presupposition that there is a gap between the causes in the form of your beliefs, desires, and other reasons, and the actual decision that you make (p. 14).But nothing fills the gap. It is itself empty.
I'm going to keep thinking about this, but I have one thought before I finish.
Zizek dedicates part of his new book, The Parallax View, to this issue. He contrasts a number of approaches to the question of how events can emerge within a materialist ontology. One theorist he discusses is, unsurprisingly, Alain Badiou, who is probably the theorist of the Event in recent decades.
Zizek dismisses Badiou for not being sufficiently materialist. This is because his Event doesn't emerge from the material coordinates of the situation that precedes it.
I'm not sure if this is correct. I understand Badiou's argument to be that Events do emerge from situations, but in a unique way. Rather than being direct consequences that are reducible to the sum of forces that precede them, they are "subtracted" from the situation. This means that the Event marks the moment when the element that had no place in that situation becomes visible.
But such an element certainly had a part of the material constitution of the situation. The critical point is that they were absent from the symbolic coordinates of that world. A good example can be found in the massive immigration protests we saw last year. People that were a physical part of their society, but were effectively erased from legitimate participation in symbolic reality made themselves known, creating a traumatic break from the world where immigrants only exist by hiding in the social margins. Its a question of their symbolic existence and I'm not sure if this is so different from Zizek's own conception of an Act that accomplishes a symbolic break (see Antigone).
Edit: To be fair, Badiou's Platonism, mathematical realism, and focus on the void as the starting point for the Event add a new twist, but I'm not convinced that those are simple idealistic gestures, especially when compared to the Lacanian argument about the Real and how the thing in itself is nothing more than the very parallax gap that separates incommensurable ways of reading the impossible deadlock that underlies the symbolic order.