Nevermind, I figured it out. I need to use MagmaSession newCommitLogSerializer to materialize the graph in a commit record, not the serializer from the current session.
However... now I'm not really sure what to make of the contents of the buffers inside the serialized buffer of the commit package. I don't see any domain objects in there, just an array of oids, the timestamp and something else that I can't seem to materialize.