[XeTeX] Malayalam typesetting using XeLaTeX assigning a key to a character

[NMPOST7]

Dear All,

How can a key be assigned to a character? On my Mac (Mountain Lion 10.5.8) I am
using a keyboard layout called KeyMagic to type in Malayalam. I got in touch the
person who created the Rachana font for Malayalam. Almost all the characters of
the alphabet have been coded excellently. Just one character which is inserted
where the vocal "a" is omitted needs to be assigned a key

The character is ഽ     and it has the coding U+0D3D

Can this be done in LaTeX? Any way to do this so that I can type the desired
character to get ഽ  .

As an example

typing -------> gives

Ta -------------> ട
pa -------------> പ


(I can copy and paste the character but that produces a place holder (dotted
circle) which does not look nice)

BR
suren