FWIW, I don't believe you have to idle in closed loop, I don't with my FAST setup. What I've learned from some tuners is that you should give the engine what it wants, not to force it. Meaning, if your engine idles smoothly and it's rich, then so be it, maybe a certain setup doesn't like to idle at stoich (as the L6 mentioned above). As long as it's safe for you and the engine, you should be ok.
I've spent way too much time trying to smooth out my Actual AF ratio vs Target AF ratio curve. I'd think with a WBO2, it's almost impossible to get the EG to idle at 14.7 each and every millisecond (or whatever your time axis is on your tables).
Just my 2cents
Owen