To get genuine “free will” or agency, you need to combine quantum physics with cognitive science. Implicitly, Zizek shows us how to do this.
From quantum physics, you need two things. First, you need there to be some minimal “free play” in matter — no saturated mechanical causality. But as everyone knows, the mere existence of non-determined phenomenon is not enough to get us real free agency, because agency is not simply a lack of determination, but implies purposefulness. Thus, the second thing you need (and here I’m “clarifying” some of Zizek’s remarks along lines suggested by Whitehead in Process and Reality) is the causal power of perception. In quantum mechanics, one is constrained to say that phenomena “notice” each other (with certain phenomena appearing “virtually” before anything else “notices”) if one is to “schematize” what is happening; and as is well known, measuring quantum phenomena directly affects those phenomena. (I’m thinking here of the third part of Indivisible Remainder, though the whole book is implied in his argument there.)
From cognitive science, you need Metzinger’s “transparent self-modeling” concept of the self. Consciousness is a “mental map” of the world; true self-consciousness arises when the map grows in complexity to the point where it contains a spot representing the self. The organism can then envision various possible chains of events and choose one (based on emotional inclination, or later, “reason”) — or as Zizek puts it, free agency is a second-order causality that allows us to choose which causal chain will determine us. Once the decision is made, of course, there is no guarantee that the organism’s design will be successful. Thus, brain size, etc., is still important to be able to accurately map the world and predict phenomena; but the shift from consciousness to self-consciousness is necessarily a qualitative, not quantitative one. The addition of this reflexive element is purely formal and virtual, adding what appears to be only a quantitative improvement to the mapping faculty, but once it is introduced, it reorganizes the “same” raw materials into a new kind of structure whereby the organism can consciously make itself do stuff, or at least try to, by means of its ability to “perceive” itself. (This obviously comes from the second part, and especially the fourth chapter, of Parallax View.)
To make this work, you need to be open to the general Schellingian idea that humanity “repeats” the primordial freedom (potentiality) at a “higher [mathematical] power.” But given that our perception is able to directly affect quantum phenomena, there seems to be some prima facie possibility of an affinity there. I would also note that nothing in this scheme requires that humans be the only ones to have made this qualitative shift; only that at least humans have done it.