Hi Nicholas,
Welcome to the Prismic community,
Well, the custom headers feature has been implemented and being tested and I will let you know once it is deployed.
Please refer to this thread for more info:
And I see your use-case to add a custom body, and I think it is a good idea so I will create a feature request and send it to our @features-team.
Please let us know if you want to have an early access to the custom headers feature.
Best,
Fares