Just an idea on how to get a complete table.

Could one not try to get booking class availability for many flights both for the operating carrier and the corresponding flights as marketing carrier?

There should be a high correlation between the availability of the marketing booking class and the mapped booking class (noting that some carriers don't show availability of more than 4 seats in a booking class). Once you have a good mix of full and empty flights, one should be able to enumerate all possibilities.