Using a LinearSVC() for multilabel classification with MultiOutputClassifier() in a pipeline in sci-kit learn
When you run your grid search, the clf step of the pipeline is replaced by each of RandomForestClassifier, LinearSVC, GaussianNB; you never actually use the MultiOutputClassifier.

You should be able to just wrap the two offending classifiers with a MultiOutputClassifier. You'll need to prefix your hyperparameters with estimator__ to get through the MOC into the underlying classifier: clf__estimator__C.