This is pretty complicated request, but a good idea.
There is two parts to this problem.
#1 sending the data to the tv. The average bike computer is a closed loop. You'd need to switch to some kind of ANT device to collect the data. And then send that data to a listener. The listener then needs to convert the data into something useful.
#2 Display on the tele. This is hackable on mythtv.
http://www.mythtv.org/
They do this with menus already, so there's at least an example of how to do an overlay in the code, but you need to spend quite a bit of time getting into the code to work it out. And that's before you've finished the listener that needs to send the data to Mythtv.
I believe there are very expensive trainers that do this already, but again, it's a closed loop system for a very good reason. It's a whole bunch of development work!!