API: how to have custom code per-mailing, not per-contact (ie news in brand template)

What we want to do is send out simple news items on our own standard email template using the API.

So I thought this would mean having a template on constant contact, and then sending a campaign with a couple of changed custom items, like the news text and the date.

So it seems perhaps we could create a campaign and save it as a draft.   Each time we want to send a news, we use the "campaign id" and "campaign activity id"  for that one campaign, and PUT the list of current recipients and do the send. 

BUT:  I don't see a way to change a piece of the content of the campaign (ie our current news item) that is not per-contact.  What am I missing?  

Thank you.