I'm not a fan of other people getting involved publicly. Talk about it privately, but keep opinions out of the public eye. They're just opinions, and as such are no more relevant than that of anybody else.
It's up to Johnson what he does. Mcgrath shouldn't single him out for criticism. It's great saying what he would do in the same circumstance, but unless you are in that situation, you can't comment. I'm fairly sure had Ferguson told Mcgrath to put the shirt on, he'd have done as he was told - end of.
You don't make a tit out of your boss and not regret it. Sometimes you have to keep quiet, and keep your head down, as your own protests won't solve the bigger problem, but simply serve to isolate you, and in many cases end your career. Dalglish will also be acting upon instruction from above. He isn't stupid. You don't get involved with matters such as racism, and represent your club, unless told to do so. It's too big a deal to act on impulse.