FWIW the site that types people's cognitive functions based on facial expression places Joan Baez as a Fi-dom. If you know your popular/folk music history, then you know she is basically the queen of activism back in the 1960's and 1970's.
Edit: I guess I'm a bit obsessed with this question because it hits home for a couple of reasons. Firstly, I have the perspective of being an extreme Ni-dom INFJ with an extreme Fi-dom sister who is about the same age, same culture, and both in the creative arts. We are both idealistic, but have also had to reconcile a lot of darkness in reality. I think she is overall the more idealistic. We both express into reality primarily through creative expression, so she enacts her idealism onto the world continuously through her visual art. That is more personal and private than social activism which is far more the domain of any extrovert than either the INFJ or INFP. We both struggle with issues of the darkness of reality.
I think there is this complex dichotomy for many idealists to struggle with desire goodness, kindness, and peace in the world, but to feel obliterated by the cruelty, falseness, and darkness. My view of reality is quite dark and every time I encounter information about suffering I want to change it - sometimes through kindness and sometimes by wishing I could be a terminator and take out the cruel people of the past. For example, I was recently reading about the neuro-psychiatrist who developed the electro-shock therapy. After I heard what he did to animals, I really wish I could go back in time and kill him. Is that idealism? Most sensitive NFs can get quite flooded by the hideousness of reality and have to cope by blocking it out in favor of viewing through rose-colored glasses, or acting upon it to try to fix things. Sometimes internal reconciliation of peace is the place to start.
In a way I'm an idealist and when younger I would influence larger groups of people by having leaderships roles in high school and college, but now my idealism mostly makes me dream of living in isolation with another hermit and just trying to survive. I think the world is hideously dark, and I try to take in reality just as it is in order to be most skilled at choosing how to make it better within the realistic limits of my person. Because the external world affects me I have to retreat far too often to be a true activist. This is a core component of the INFJ descriptions - that need to suddenly retreat. I think it's the Fe-doms and Te-doms (based on Jung's descriptions) that are far more likely to act their internal ideas onto the world.