new entities?

NZ Maori and several other polynesian languages use a latin alphabet 
with macronised vowels. A macron is a straight line over a letter 
rather like this:
_
O

and it usually indicates a lengthened vowel sound. Would it be 
difficult for macronised vowels to be included in future HTML entity 
sets? Or do macrons first have to be accepted by some other standards 
group that defines character sets? I know that font support for macrons 
is likely to remain ad-hoc and regionally limited.

I imagine the required entities would be something like &Amacron;, 
&Emacron;... or &Amac;, &Emac... 

thanks,


Douglas Bagnall