Hmm, not familiar enough with those specific woods to give accurate advice.
Bark is lost by the shrinking and expanding of the wood. Basically the wood drys up and shrinks, the bark then has nothing to adhere to and falls off.
I have seen the following methods. 2 Part epoxy, cut the bark off using a band saw, then adhere it back in place. It's a time consuming method but will work. Make sure to use wood clamps to secure the bark back in place. Again, make sure you do this while the wood is fairly dry.
Epoxy the whole piece. This basically preserves the whole piece and should be less resistant to moisture changes. However, you need to make sure you do the whole piece, and you have a good coat.
Oops I lied... Its Epoxy over the whole thing not polyurethane.
Epoxy is used a lot to make a glass type look.
Here is an example.
http://www.pkendall.com/wp-content/uploads/2010/02/Kit6.jpg
Good luck! I would do a test piece with the two part epoxy if you have access to a band saw.