I think a mortise and tenon joint would actually be hidden pretty well if you could have a tight fit and a finger joint/dovetail wouldn't look all that bad if it were visible. But, geez, the only way I can think of joining the material would be to do the tenon joint on an actual blank and then somehow cut the scales out of it. It's beyond my scope to figure out how to connect each piece of the nut together.
Try sending Bill Ellis or Joe Chandler an email or PM if you haven't done so, and they might be willing to take the job on, or at least suggest a different material that would look like the tagua nut but would be more practical.