I'm a long time user of the Boss OC-3. In trying to create that effect on the helix, I've tried the split-crossover trick and other drastic EQing but no matter what you can still hear the sub octave in the entire range of the guitar.
For now, I'm just going to stick the OC-3 (a reasonably cheap and small pedal) in the fx loop.
The Electroharmonix Sub'n'up pedal does the same trick. You can only access that parameter through their toneprint software though. I've heard its tracking is even better than the OC-3.
Hopefully there will be one of these pedals modeled for a firmware update!