Dear Tom,
Yes, the DimensionSegment is not element so you cannot delete it via Document.Delete API.
I am sorry to say that there is currently no API access to this functionality.
You may have to consider to rebuild the dimension as workaround, for example, get all references of Dimension via Dimension.Reference, and then re-create new dimensions, remove the old dimension lastly.
It's recommended to post your new API request in official vote forum below:
http://forums.autodesk.com/t5/revit-ideas/idb-p/302/tab/most-recent
Note that this issue needs to be assessed by our engineering team, and prioritized against all of the other change requests that are outstanding. As a result any information that you can provide to influence this assessment will help. Please provide the following where possible:
• Impact on your application and/or your development.
• The number of users affected.
• The potential revenue impact to you.
• The potential revenue impact to Autodesk.
• Realistic timescale over which a fix would help you.
• In the case of a request for a new feature or a feature enhancement, please also provide detailed Use Cases for the workflows that this change would address.
This information is extremely important. Our engineering team have limited resources, and so must focus their efforts on the highest impact Change Requests. We do understand that this will cause you delays and affect your development planning, and we appreciate your cooperation and patience.
Jim Jia
Autodesk Forge Evangelist
https://forge.autodesk.com
Developer Technical Services
Autodesk Developer Network
Email: Jim.Jia@autodesk.com