A couple of you had asked me about this and I am glad it turned out to be reasonably easy to implement.
I have updated the cm transliteration editor at http://arunk.freepgs.com/cmtranslit
with a new feature that allows you to paste already created tamil/kannada/telugu/sanskrit script. Once you hit the Translate button, this will convert it into the transliteration scheme (results under English tab) as well as to other languages.
You can use this as a convenient "quick start" or "starting point" to get something info the unified scheme - particularly if you already have text in an Indic language or you find it convenient to create it elsewhere. Once you do that you can copy the results under the English tab, modify it as necessary and do further tweaking.
A few points to note:
1. The feature is experimental and so there may be bugs
2. The feature is not 100% reliable. From tamizh script it is not possible to unambuously figure out ka vs ga, pa vs ba etc. So always check the English results and modify it as necessary. The same thing applies for anuswara. While kannada and telugu always use anuswara in some contexts (and hence no ambiguity), Sanskrit doesnt. So conversion of anuswaras using this feature may end being less than satisfactory.
Pl. let me know of bugs and any improvements.