Customizing the extent of the ignore does sound appropriate.
As for the dangers of the list contents being uncovered.. I'll say the risk is nonexistant. Why? Because, at least in the current conceptual incarnation, attempting to reply to the ignorer (if you're being ignored) will notify you. That aspect I'm firm on - I don't understand ignore/block lists that don't notify the person being ignored or blocked. Without it, the person won't ever realize the situation and would continue to erroneously think that their messages are being read by the recipient. That's just inane and provides no closure whatsoever.
Even then, someone can communicate to others that they're being ignored by so-and-so. It's no real secret. Also, odds are the person hitting the ignore button will also make a proclamation as their "last word" that essentially boils down to, "I'm going to ignore you now!".
The idea was only conceived tonight. Give it a little time to evolve. :)