Nearly 5 years since Quaternion BP support was requested by multiple people

See this post with Epic employees promising new additions to the BP scripting interface for Quaternions in 2014: Quaternion in blueprint - Feedback for Unreal Engine team - Unreal Engine Forums

If this isn’t on the roadmap please treat this post as a feature request to put it on it.

4.22 has exposed a lot more math support, including for Quaternions. Feel free to take a look at the preview.