This is one answer to the question (and probably not the best) and just sort of a shot in the dark. Having not seen the tree, only hearing the description, I offer this simple answer. This could be done as the tree is being styled and made ready for a pot, the 'fake trunk' can be cut, carved and treated with enough preservative to make it last for years (once 'planted').

This solution will never make a great tree, it will only add interest to a tree that has a nice top and could be worked further as a garden display piece.

1. I would style the tree beforehand and ignore the bad taper until it's potted
2. Use a piece of Juniper, cypress or similar wood for color, grain and longevity
3. With the 'solution' in mind, you can take time to select and work the right piece of driftwood to make it ready
4. While styling and selecting, the 'fake trunk' can be refined in shape and size to be as interesting as possible. Fake roots, jins, etc. You should make it 'artistically' part of the tree and it's design
5. Once finished (tree, pot, fake trunk) always assure people that it was the way you found it: "Oh, all I did was clean it up and add the Lime Sulfur, pity though, the other trunk would have been so much nicer." (And smile knowingly as you say that)

I hope that this was some help. Remember, make the 'tree story' good if you do it this way!