If the clear mirror is 'when self expands to include everyone and everything', it's a concept we can aim for but can't truly know. Just like we can't actually be aware of being buddha.
For one thing we can't ever truly know where another person is coming from or what the consequences of our actions might be.
For me right now, the clear mirror is another mirror which I can use to evaluate my own actions retroactively -- even when that means having to face my own ineffectiveness and/or ineptitude with equanimity. In the concept of the clear mirror, we're not only accepting everyone else for who they are, but ourselves.
Since reading Kokyo several months ago, I've been trying to identify actions that I've witnessed or experienced that might resemble the actions of past Zen masters. At least in the States, disciplinary action has come under scrutiny and people come from more diverse backgrounds. One of the few instances I can think of occurred at Antaiji, where during one zazen period, Muho-san yelled with his booming Zen master voice (at me, since he yelled in English) to "Stop Moving!" ..... In that instance Muho-san was wrong and I got to respond by immediately ringing the bell since I was Jikido that day. I can think of no other instance involving Muho-san during the three months I was at Antaiji. That's not to say that my behavior or anyone else's was perfect. Rather, from what I know, dramatic actions may be becoming more rare these days, even in monasteries. That might be a good thing since life can be traumatic enough.
Personally, I can say that I've typically most appreciated those people who have been present with the process each of us goes through in our practice. People who have offered, not judgement, but the occasional word of encouragement and hint if correction was warranted.