Hi @anon66726932,
I have news on this, but I’m afraid it’s not good news.
You and I were correct that Asana is storing custom field values within the Status Update object. Unfortunately those values are not accessible via the API for either reading or writing.
The API team has added a feature request to be able to access those. But that just means it’s now on a long backlog of API enhancements to make, so I would not count on it being added any time soon.