If I’m understanding this suggestion, I think it is technically possible to add a list_nns_neurons method to SNS governance. Not sure what actual problem that solves. In other words, this might be a YAGNI feature.
list_nns_neurons would not actually force an SNS to make its neurons public, because an SNS can do what GOLDDAO did: they can create another canister to be the controller of their NNS neurons. Then, list_nns_neurons would return nothing.
At the same time, SNSs that actually want to make their neurons public can do that already without adding list_nns_neurons to SNS governance. One option is for the SNS to set the visibility of its NNS neurons to Public.