Is Christian Time Travel Possible?

I've just finished Madeleine L'Engle's An Acceptable Time, the last novel in her Time series, which I picked up on the recommendations of Enbrethiliel and Lissla Lissar in the combox here.  I absolutely loved it.   At the same time, though I still haven't read his Landscape with Dragons, I'm starting to get an idea of why Michael D. O'Brien might have a bit of a beef with L'Engle.  The novel is about Meg Murry and Calvin O'Keefe's oldest daughter, Polly, who goes to stay with her grandparents, the celebrated Dr. and Dr. Murry, in rural Connecticut, in order to be homeschooled.  (Now, you'd think that's a premise that a self-styled guardian of traditional Catholic mores ought to love, but the homeschooling has been proposed as a solution to the fact that there are drugs and riotous living at her high school, and besides, like all the Murrys and their descendants, she's absolutely freaking brilliant, and so is poorly served by her district school; so there's no whiff of religious orthodoxy in the equation.)  Like many of the Murry-O'Keefes, Polly finds that she can time-travel, and she has some gripping adventures on the site of her grandparents' farm three thousand years ago.  She has a companion in her time travels, a retired Episcopalian bishop, who, though he unashamedly and frequently professes Christ, appears as a sort of Teilhard de Chardin figure. (Now, here's a bit of trivia for you:  I've actully been in the New York apartment in which Teilhard died in 1955.  It is in a building -- in the neighborhood where L'Engle herself lived, the neighborhood she writes about in The Young Unicorns -- that was once a Jesuit residence, and one of my professors from grad school lives there now. I wonder if L'Engle knew Teilhard or his confrères?)

Polly and Bishop Colubra find themselves in extreme danger three thousand years ago, and are unable to transfer themselves back to the present.  While Polly has more faith in science than in God, the Bishop invokes the mercy of Christ in ways both subtle and obvious -- intoning the Kyrie when he's been captured, for instance, and assuring Polly that Christ existed even a thousand years before His birth.  However, he also believes that the ancient druids possessed knowledge that has now been lost -- knowledge, for instance, of the techniques of time travel -- and I suppose that it is these statements that a critic like O'Brien would call heretical.

Now, I don't want to set up a straw man, because I haven't read O'Brien's polemic (though I do have it on order through interlibrary loan).  If anyone has read the book and can set me straight, please do so in the combox.  But it seems to me that, if you believe that we exist in both time and in eternity (which is what we believe, isn't it?), it is not heretical to believe in the possibility of time travel.  We already know that it is good and meet to pray for the dead as if they were still alive.  Nor is it heretical, is it, to suggest that ancient peoples and pre-Christian cultures possessed knowledge that has been lost to us?  Can't one believe in Jesus Christ as the Lord of and the fulfillment of human history and, at the same time, acknowledge that non-Christian lands and people had a great deal to offer?

I suppose the problem comes when we try to resurrect lost knowledge in its original context, rather than baptizing it in the light of the salvific proclamation.  Of course, we should not try to use any power to circumvent or second-guess the will of God, and part of the discipline of Christianity for some who have the special gifts of which Saint Paul speaks is the laying aside of those abilities in favor of accepting smallness, weakness, and utter dependence on God.  But, contra O'Brien, I don't believe we should be afraid to read any literary suggestions of special gifts existing in a non-Christian context, or about time travel, or about snakes or dragons who are good rather than evil.