Is it reasonable to expect me to necessarily be able to answer that question if materialism is true?

It's about as reasonable as demanding an account of how the brain can mantain mesoscopic quantum superpositions long enough to influence neural processes.

(What does the mapping from entangled quantum states to experiences look like?)

We don't know, but it's going to be far simpler than a mapping from classical brains.

(For all we know, it could be trivial; perhaps each quale is a GENSYM which maps directly to a basis of the quantum system.)