To create a data-only document, I have not used C#/.NET, but have it working from Visual Foxpro (or any language that can manipulate an HTTP request object).
The resource in play is this one:
and then once I have an HTTP request object instance established (in this case, "_DWPlatformService"), the code to POST the request looks like this:
* Need to use a Content-Type of "application/xml" otherwise the POST will simply create a document with the index data XML inside it.
The POST data looks something like this, for a cabinet including indexes COMPANY and SENDER as an example:
Data Only Company
Data Only Sender
This will create a data-only document in the desired file cabinet.