What specifically are you streaming? Do you mean for downscales? There is a way around this -- you can set the base resolution to the desired resolution, then hit Ctrl-F on the source when it's selected to make it "fit" to that screen, but it will forfeit proper downscale filtering. Hmm, then again, if you want proper downscale filtering with that output, you can "hack" a custom downscale resolution by doing the same, but instead doing something that is 1.5 or 2.0 times the size of 612, then using the downscale.
For example, let's say your output is indeed 1680x1050, and you want to output at 979x612, yet you still want properly filtered downscale output. You could instead theoretically set your main resolution to 1958x1224, use a 2.0 downscale, then use the "fit to screen" on your source to make it fit to the screen. It should then give proper downscaled filtering to 612p.
Properly filtered downscaling is not a simple process - that's why there are only a few downscale selections.