Non-expert comments cont'd:

Paraffin wax is an interesting idea, since it will soak into the leather's pores. And you have a firestarting candle/handle with you at all times (I kid, I kid). But I wonder if its waterproofing would extend to the carbon steel tang in a consistent way (think freeze/thaw and humidity changes).

On the other hand, what about other modern materials? Last fall, I sealed some windows with GE exterior grade silicone that has a 35 year warranty. A knife handle is a fairly rigid structure; could this be glue and sealant all in one?