Here is a more full context:
May we not, by a reasonable analogy, suppose likewise that there is no
experience of the spirit so transcendent and supernatural, no vision
of Deity Himself so close and so far beyond all images and emotions,
that to it also there cannot be an appropriate correspondence on the
sensory level? Not by a new sense but by the incredible flooding of
those very sensations we now have with a meaning, a transvaluation, of
which we have here no faintest guess?
To me it seems as if the word "meaning" here is being used as in "signification". As in, "What does it mean?" Or, "What does it signify?" The word "transvaluation" means literally, "a valuation beyond". The dictionary has in its definition the idea of using a standard to evaluate that is not the normal one. Lewis is saying here that our senses will have experiences in correspondence with what will be in heaven, but they will have a meaning that is on a whole different scale from what we experience now.
That it does not mean "purpose" or "reason" in this context, I think we can gather from the fact that Lewis is talking about what will, at the time of the flooding of "those very sensations we now have", be in the present. He is not talking about purpose, I don't think, or telos. That is, he is talking about what our present sensations will mean at some future time. But that future time will not, in itself, be pointing even further forward in time, as if it were a reason for something else. Indeed, time will have come to an end when this flooding happens!