Hi @Oetze van den Broek I appreciate it if you can post this question on to the GitHub repository page The product team are actively monitoring the issues there, and you would get direct feedback and assistance from the PMs and Dev team.
Hope this helps!
Update
This feature is not supported with the current implementation. A Product Feedback request has been logged to address this request. This feedback request can be upvoted through the URL - Allow private dtdl repositories for ADU
If the response helped, please do click Accept Answer and Yes for the answer provided. Doing so would help other community members with similar issue identify the solution. I highly appreciate your contribution to the community.