Auto-generated namespace in (Cluster)RoleBindings

[cschmatzler]

According to the Tanka documentation:

Tanka sets metadata.namespace to the value of spec.namespace in spec.json

That works great, and creates all resources, including serviceaccounts, in that spec.json namespace.
When creating a RoleBinding, the subject option requires a namespace to be set. How do I get that in there? Right now, I'm adding an extra namespace option to the jsonnet config, but that kinda defeats the purpose of Tanka adding it automatically.
Is there a way to refer to that spec.json namespace inside a RoleBinding subject?