#1769 introduced the ability to use set_params for setting the elements of a Pipeline/FeatureUnion with parameter search. It also allowed transformers to be removed by setting to None. Some of this functionality is mediated by a new base class sklearn.pipeline._BasePipeline.

For completeness, these features should be extended to VotingClassifier (although it's not as clear how useful they'll be there; setting to None can be accomplished with the weights parameter). Certainly the step name validation introduced by #1769 should be adopted in VotingClassifier (see also bug #1800).

It may be worthwhile to rename and/or move _BasePipeline in the process.