Just like Custom field values will transfer from ideas to Feature/Master Features/Initiatives when promoted assuming a custom field with the same Key name exists in both, the same should occur with many to many custom fields as well. My use case is as follows:
1 - I add a Customers custom table. I add it to both ideas and to features as a Many to many relationship type.
2 - I select a few customer values for an idea.
3- I promote the Idea to become a feature.
I expected/hoped that the customer values would populate the corresponding feature since the same table is being used in both. That did not happen. I would expect this to be a common use case associated with Promoting ideas.