Tony,
The reason I posted this question to Aikiweb is to get the input of a ready pool of aikido practitioners, versus woodworkers; but yes, it may be better to ask my friendly neighborhood woodworker. We've taken a look at some local woods; the typical plant material of choice for martial arts training weapons here seems to be rattan, which may be a bit light for our purposes. However, your mention of "merbau" has certainly piqued my interest; there seems to be a wood of the same name around here...hopefully it's the same stuff you're describing. I'll also take a look at eucalyptus and the rosewood (although rosewood may be too pretty to abuse).
Thank you, Tony.
Regards,
John Kuo