20 March 2018
Docentric Toolkit also has a so called "Template Management" APIs that allow you to create and update template documents. But the problem is that the current version only allows you to programmatically change a data source schema or preview data and there is no way to modify tagging elements (e.g. data bindings).
I think the easiest way to do it is, as you already pointed to, by opening these templates as OOXML packages and finding Docentric Toolkit "Template Metadata" (or Project Data). This is an XML document embedded in a template document as a custom XML part and you will find it inside folder "customXML". Normally the name of the file/part is "Item1.xml" but it can be anything else. Just open it and you will probably guess what to do if you need to do some basic find/replace operation.
I strongly suggest that you install Open XML Package Editor for Visual Studio. It will help you a lot with editing Open XML packages. Just note that it doesn't work with the latest VS2017.
I hope this helps,