Before I test anything, I want to give my thoughts on it, which might contain some pointers.
You can use the "Quick and dirty" method, which will route the sound through ALSA. If I had to guess, this should be fairly easy to set up in OBS.
If you want the JACK way, you'd have to find a way to make OBS accept JACK as input. Start the game, open Catia and tie Rocksmith's output to the OBS sink too. (See here and imagine there's an input sink for OBS)
Otherwise, you can still grab it from your physical audio jack, but that's just not elegant and not ideal for streaming - Or via capture card, if you have one.
I'll have a look into it. This might be a good topic for another page in the guide.