Harry,
As Hans suggested earlier, m̆ (m + Unicode Character 'COMBINING BREVE' (U+0306)) is used for realizing m with chandra over, in many of the recent linguistic books dealing with Sinhala. Similarly, n̆ is used for n with chandra over. 

Yes, you can submit proposals for new character and combining characters for Unicode Technical Committee (UTC)  to consider. You may find the instructions in the weblink below useful:

http://unicode.org/pending/proposals.html

Regards,
Suresh.



On Wed, Aug 10, 2016 at 3:31 PM, Harry Spier <hspier.muktabodha@gmail.com> wrote:
Thank you for this information Suresh.

Suresh wrote:

  1. Please don't use ṁ for transcribing the half nasal. ṁ is used for anusvāra in ISO-15919. Sinhala should be using ISO-15919, not IAST. ṃ is not used in ISO-15919.
  2. ...
  3. m̐ should serve your purposes for representing the labial prenasalized consonant ඹ (U+0DB9 SINHALA LETTER AMBA BAYANNA)
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This is of course a workable work-around but the ISO 15919 standard for this letter is m with chandra over it not chandrabindu.  Suresh can you confirm if there is a unicode letter of m with chandra over it.  Is there a procedure to add new letters to Unicode.  I would think its reasonable that all ISO 15919 letters were included in Unicode as single glyphs.

Thanks,
Harry Spier