I think regular education about mutual aid would be helpful. For example, while anonymity does mean capacity for harm and then reacting meta posts, look at what happens when donating to, e.g., charities: you end up mostly paying for PMC paychecks and tax breaks for corpos. Edit: maybe adding an explainer comment pinned in every mutual aid post would be a good way to implement this.
I do think that having a vouching system is the best way and my understanding is that this is basically the status quo to some extent. You may want to define some kind of unvouching process in cases where there are verifiable issues. I think this will actually increase trust and donations if communication and messaging are on-point.
So re: meta posts, I think there needs to be more process, not just allowing meta posts and comments. This requires more mod intervention, so I'm sorry to be basically volunteering others' time, but I do think it will be beneficial to have meta posts go through a filter and deliberative process and with clear results and strategic messaging. The alternative, i.e. status quo, is likely to be ad hoc discouragement. The coupling of "this is why you should keep helping our comrades" to any meta posts is very important.