First, a request (generated this time by Quatermass's reference to The Doctor)--if any of us are referring to a character who is totally outside the Pratchett cannon, please identify them clearly and explain for those of us not familiar with that book/character why they are relevant. Tony explained that this was a reference, he said, to Dr. Who--a series I'm not familiar with.
Going back to the question of Lu Tze, or in fact to the first pages of the book--it is, in fact, the 493rd Abbot who sends him to deal with the "problem of Omnia". Clearly, Terry hasn't fully developed his ideas about the role of the History Monks, and even more about Lu Tze, in this book. He is described, in this opening passage, as one of the Abbot's most senior monksrather than as the sweeper role he plays in every other book. And he is a deus ex machina (even more than the Abbot intended) in making sure that Om not only survives but that he and Brutha are jointly educated by each other.
I think that Terry is using the monks in this book really as a plot device--a machine that makes sure that a certain small god survives and learns something. I think that Terry is really trying to explore religions and philosophy (not the nature of Time). But the question of Time and certain individual characters (or in at least one case deceased characters) is something that Terry begins to see more and more as a significant factor. But here, he is more concerned with the nature of gods, religions, and human beings (especially who creates whom).
And one last question to throw out for thought--how does Brutha become wise enough to encompass the material of the library and thus to create a somewhat better church. But, we know from later books, that the church has suffered from different problems after Brutha's death.