• Failed attempts could be seen at experiment-n-* branches
• Working solutions from tiki and from Melvyn are located on solutions-{user} branches.

About solutions:

• Tiki solution works for serializer, but fails to provide correct message for model. It was not a requirement of an original question, but it is worth mentioning.
• Melvyn solution works both for serializer and model errors.

Apparently the ModelSerializer doesn't copy the error_messages attribute from the model field, so we have to do that by hand.

• Melvyn