Dear Rolf and others,

Sorry for a late response, but I just saw this thread. Since I have some experience with Unicode as a member of the Unicode Consortium, Here are my quick remarks:

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. I don't know what work/application you need this symbol for, but the use of combining diacritical marks are very common in Unicode, and widely used for Indological purposes, as Hans pointed out. 
3. m̐ should serve your purposes for representing the labial prenasalized consonant ඹ (U+0DB9 SINHALA LETTER AMBA BAYANNA)
   
4. In the world of Unicode, nobody should be bothering about individual glyphs. If you work requires it, then, it is perhaps using an outdated technology. Abandon it :).
5. If you use Unicode, it should not matter whether you are using a Mac, PC or any other publishing software. The text should be as transportable as the ordinary English Text (which follows ASCII standard). Unicode is an international standard, and almost all the applications and operating systems developed in the last decade should fully support it.

Regards,

Suresh.