Teaching pronunciation is teaching listening to speech to hear new sounds and patterns so that we can change the learner's values of perception to enable a better understanding of the voices they hear.
It's a process of helping a learner adjust their values.
It's broadening their sound spectrum just a bit with every lesson so they are more independent and more conscious of the choices they make.
Listening and comparing are the chief activities. Output should be used as a means of communicating a change in the learner's ability to perceive.
Richard Schmidt wrote nicely about consciousness in the language learning process. Here's a link to one his papers I like.
So a lesson can be judged as successful if a learner can distinguish between two versions of a speech signal. It can be judged as having been beneficial because they have developed a new awareness. New awareness is the first step in learning.
And learning is not changing but thinking in a new and more useful way about a task.
Record those learners. And let them listen to recordings. Let them listen to recordings of other students.
Aid the listen process with visuals and interactivities. And by having conversations and encouraging play with how people speak.
It's all still unexplored. Try something this week.
Remember that the goal is not a native like accent but simply clear communication.