You don't have to stream 16:9. Let me say it again, you don't have to stream 16:9. I've had to tell this to people countless times because they think that they -have- to stream at the same aspect ratio that their viewers commonly watch at. But you don't. Your viewers complain about black bars? It's because they lack the simple knowledge that not everyone has a 16:9 monitor. They complain, just simply tell them that you don't have a 16:9 monitor. If they continue to complain after you tell them that, then they are entitled jerks and aren't worth keeping as a viewer. Do you -seriously- want to stretch your image just to satisfy such jerks?
You -can- stretch the image of course, you just set the base resolution to whatever the heck you want and stretch the source.