Digging through the repository looking for any code that deals with handling emails, I eventually found the rake task posts:refresh_emails which takes an optional topic ID. Using the following, as the name implies, reprocessed the emails and also rebaked all the posts in the specified topic.
cd /var/discourse
./launcher enter app
rake posts:refresh_emails['24961']
This provided the posts in that topic untrimmed as expected with trim incoming emails disabled. That did expose another issue I need to deal with before I can proceed but that’s not for this topic. To refresh all emails across all topics, I will be able to use the rake task as follows.
cd /var/discourse
./launcher enter app
rake posts:refresh_emails