Thanks to Nick and Jambunathan I have got a minimal setup to be able to
type in English (roman script) and easily transliterate to Sanskrit
(Devanagari).

Now I am exploring how I could 'zip' the two together. My requirements are
like this:

I will be teaching singing to a mixed group using a projector.  Those who
can read sanskrit would be put off by the roman (English) and those who
cant of course need the roman.
The attached screenshot shows two emacs buffers side-by-side with the two
versions.

I am now exploring the possibilities of how to make a 'presentation'
putting the two together.
I am not too comfortable using emacs for the final show because emacs
occasionally crashes -- due to non-standard fonts, input methods or what I
dont know -- and I dont want this to happen in front of 50 people!

Any thoughts/suggestions?

Rusi