Spikey,
My HOG works fine with the Helix, and I'd guess that the pitch tracking algorithms are similar (if not identical) to those used in the 9 series. The key is sending a fairly clean signal to the unit, without chorus/phaser/reverb/etc - it gets pretty funky if you do that. I typically do a send to the HOG as the 1st or 2nd block (1st might be compression), and then return it somewhere later on in the chain where it makes most sense for the sound I want.
Haven't tried it after a ring mod - that might be interesting.
For external stompboxes, use "Instrument" level for send/returns. Basically, anything designed to have an instrument directly plugged in (like a stombox) is designed at instrument level.