What a great idea! Please let me know if you realize we are missing enablement resources on Experience League (e.g. tutorials, videos) that are important for your group to master Platform!

Yes, there is a UI limitation right now in that the tags interface only shows the identity namespaces from the Prod sandbox. If you are able to duplicate the same namespace in your Prod sandbox it will then appear and you can select it and use it with requests made to your dev sandbox.