Morning Matt,
When turning Ivory piano keys make sure your tools are sharp, TCT would be better.
Edit.. The only thing I would say is make sure you have proof that the Ivory pens are turned from piano keys and you have proof of where they have come from. Otherwise you might find that the eyes of the law will be upon you. It's ok converting ivory provided you can prove its origin and not been obtained since the ban.
But in all honesty you will probably end up making the pen and keeping it. Not many want then now adays with the stigma attached. Only my opinion. Finished Edit..
As for Walnut, that turns easily and looks nice on pens.
Why don't you put a post on here my friend.
http://www.penturners.co.uk/forumdisplay.php?f=11
It lets us get to know "Matt" and what you are and do. Always breaks down barriers my friend and gives us a better understanding on how to answer questions.