I didn’t know about this Git feature until now. Seems likely that this isn’t supported. You might want to make a feature request: https://support.github.com/contact/feedback
Support tickets can be found at https://support.github.com/tickets/
But not every inquiry ends up there. I honestly don’t follow what is tracked there and what isn’t. It might be limited to requests you send via a customer account.
I would be mindful of using niche features like Git namespaces with a service like GitHub. Although GitHub may incidentally support them, it’s unlikely that it will ever be a high priority and when a new product like Actions has so many high-impact feature requests, it’s unlikely anything that isn’t a high priority will be implemented. How important are Git namespaces to your workflow? You can see the public GitHub roadmap and filter by Actions to see how much work is already planned.
Sure, this is quite a specific feature. But these kinds of features open new scenarios that might create new types of distributed applications with Git under the hood.
I’m working on one of these applications - https://dvc.org - and the advance Git and GitHub features would help a lot in building a better user experience.