Golly. This is a new record. I’ve fixed it.
I spent FOUR MONTHS on this problem, registered for this forum specially…
..and solved it myself within 5 mins of registering.
The syntax is:
--meta_data='[{"key": "test_meta1","value": "test_meta_value"}]'
eg
sudo wp wc product create --name="test4" --regular_price=10
--meta_data='[{"key": "test_meta1","value": "test_meta_value"}]' --user=1
I better go and close my own stackoverflow question.