[XeTeX] Diacritical accent marks using \.{}, \d{} in Tamil/Malayalam

[P. P. Narayanaswami]

I need to put a diacritical dot over a character, and
a dot under another character in Tamil/malayalam.  
The usual TeX commands \.{}  and \d{} do not seem to work here,
even if I use double parenthesis \.{{ }}  or \d{{ }}.
The followinmg does not work

\def\tam{\fontspec[Script=Tamil]{Latha}}
\tam 
\d{{\tam   ரி}}  \.{{\tam ரி }}

Any help will be appreciated.  Thanks in advance.