Thanks for the suggestion ! We'll have to look into the EPG, for now we get the TV listings data in another way ;)
Right now we only have 2 STBs, and their input is captured by 2 video cards (http://www.blackmagic-design.com/products/intensity/). We picked this one because it has a very nice C++ SDK. Ideally though, we'd have as many STBs as we can to get more screenshots !