Hi Vandoo! I'm here to tell you that you're right about the dugong ivory! I found a very nicely made 18th century Chinese bracelet made with pieces of dugong "tusks" (incisors), but can't for the life of me find the reference now. - I'll check into this topic again when I find it!
In the meantime, here is a native Australian piece from a collection at the University of Glasgow:
http://www.huntsearch.gla.ac.uk/cgi-...&browseMode=on
This is a much more crude piece than aforementioned Chinese bracelet, but it does however show that dugong ivory is both practically usable and that it indeed has a history of use in several cultures. As you yourself pointed out back in post No. #5 however, the size and shape of these "tusks" are not suitable for mandau handles.
So to recap, these are the standing questions atm:
1): Has bone been used as a source of mandau handle material, as we see it with sambar stag and wood?
2): IF bone has been used as handle material, which animal species did then supply the bone?
This is SO exciting I think, and I feel confident that we will get around it in due time.
All the best, - Thor