I did some experimentation, and if set so that it is fully (or possibly nearly) heal out with zero float, I could release it in. BUT, if you have it set to be neutral or heel in, then the spindle will hit the plate. It's not just a matter of being hard to do, it just physically cannot rotate enough to release. I have mine centered with zero float, so mine cannot release in.
Okay...I see what you are saying. If you are set up with any toe-out at all then clipping out heel-in may not be possible with Zeros. I pedal exactly neutral, no toe-out or toe-in and I am able to unclip heel-in with both feet.
You are the man!