It’s not. They basically did everything you would need with the favorites system. The mounts are not individual items exactly, they are likely classes of items. You wouldn’t have to do it by hand. Blizzards side would not be much more work than a full toggle. You could even do it without adding extra data if you leverage a Null value for it.
Biggest thing would be adding a second check box. Just get rid of favorites and turn it into static flying favorite and dynamic flying favorite. If neither is set the mount uses their default flying value which is already set.
From a programming / tech stand point at the very worst it would be a few extra booleans. I think it can be done without adding any extra data though.
Oh and currently favorites are server wide so don’t think that would have to change here. Just tie it into that system.