I think there are standard and non-standard forms of materialism. I consider a form of materialism to be standard if it holds to three theses.
1) Reality at the basic level is mechanistic. There is no teleology, no intentionality, no subjectivity, and no normativity at the basic level.
2) The basic level (typically called physics) is causally closed.
3) Higher level states (such as the biological, the mental, or the sociological) supervene upon the physical. Given the physical, the higher level states must be exactly as they are.
It seems to me that if you accept this picture, nothing like libertarian free will can be coherently maintained. Now if one is trying to call oneself a materialist and reject part of this picture, in other words, if one adopts a non-standard form of materialism, in virtue of the fact that, say, everything in the mind has a spatial location, then you might be able to find room for libertarian free will.
Which always makes we wonder about Christians, like van Inwagen, who call themselves materialists. Do they really conform to the standard definition of materialism?