I know this is an old thread, but last night I was curious if this would work, using the Helix as an interface to my pc for RS, and I found this thread. Looking like it was a dead end, I started playing with it, and I got it to work perfectly.
I created a "rocksmith" patch with two paths, one with my amp sims and distortion with an output to my XLRs, then a second clean path with output to the USB 1/2 out. Plugged the Helix LT into my pc with the normal USB A-B cable, selected input microphone in my PC audio settings, speaker output as my tv, and then set up RS to use the microphone instead of the real tone cable. Worked perfectly. No lag that I could notice, plus I'm using the Helix for the guitar sounds but I still get the music and background "backing tracks" from RS through to my TV speakers. I could have also routed the RS to my XLR's by adding a send/return loop to the first path.
Maybe this will help someone else looking to do this.