Hello all,
Back to your problem, i think most cable tv operators do this (using proprietary software and/or hardware equipment), so perhaps consulting a few of them would be a good idea to get started.
Last year we had a session on pure data in sarai, and he was using webcam streaming with tickers. One can use pure-data on all platforms check their website at http://puredata.info/ Though I am not sure how will that fit with Tata-sky.
Yes, not sure how you'd get the stream in from Tata-Sky and re-broadcast out -- my guess is you would need something like a TV tuner card on the machine, get video in, composite text over, and send it back out to a television over S-Video / VGA / HDMI . I'm fairly certain this should be possible, if this is the setup you're looking at. I guess pure-data should be able to do it. Another software I'd look at is FreeJ [1] - I have done this kind of things of putting text over videos live and playing them on a projector - works quite good -- not sure how exactly it might interface with the hardware you require, but I have heard of people using FreeJ with TV tuner cards, etc. to do a lot more complex things and performances. If you haven't bought additional hardware yet, it may be a good idea to ask on the FreeJ list for recommended hardware, etc for this kind of setup.
I think FreeJ might be the best solution - the developers are also quite helpful and run an active mailing list and IRC room.
Saying this, it may also be possible to do this without a software like FreeJ and just with ffmpeg or something to do text overlays. However, if you are looking to animate the text to be scrolling, etc. - the tool I've seen that does this really well is FreeJ (look for details on the TextLayer .. is totally scriptable using python or javascript [2], and can be lots of fun once you get through possible initial hurdles of hardware compatibility / codec compatibility, etc. :)).
Best've luck, Sanjay
[1] FreeJ Vision Mixer - http://freej.org [2] FreeJ Scripting Documentation - http://freej.dyne.org/docs/scripting/