Hi nallenscott,
The format of the HTTP publisher is not modifiable. You can add static parameters, but that's it.
What is available is the HttpBuilder script utility. see: https://help.infiniteautomation.com/about-mango-java-script or the contextual help for scripting within Mango. There are a few example uses around the forum: https://www.google.com/search?q=site%3A%2F%2Fforum.infiniteautomation.com+httpbuilder&oq=site%3A%2F%2Fforum.infiniteautomation.com+httpbuilder
You could do this in a script that is triggered on context update. The only downside here would be the HTTP Publisher is expecting a 200 OK back or it will attempt to publish the value again later, but you could script that if you required it by storing the update in some JavaScript list within the script.