Whatever method is used for the cable boxes should be used for the TVs as well.
phil's low tech method is great if the walls are not roo reflective. A dark room would be great for this.
phil's high tech method would require you to use a remote that would issue two commands for each button push -- a)which cable box/TV it was calling to and b)the actual command.

Another approach I used in a sports bar with seven DSS receivers was to put seven (three in this case)IR receivers in, each at the back of a rectangular tube shaped so you can stick the end of the remote into it and then push the remote's buttons.
The drawback of this approach is that you can only use the remote in one location, where you put these tubes. In a sports bar, that's easy; behind the bar.