I think that my imagining of the tool is sufficiently different to think that it isn't the same tool at all, and different workspaces wouldn't even be a thing. The composer UI in this case would provide a way to logically separate infra into different code bases but it would still have to know about all of them.

Edit: and actually, without a state there is no concept of a remote state. All codebases talk to the same "service" so circling a bunch of infrastructure and saving it to a specific git project would be easier than connecting to a bunch of remote states and reading specific resource days from them. Even if you're code repo doesn't declare a resource, the omniscient IAC service "knows all"

TF workspaces are one of the most complained about features at my company, hopefully a new tool would do something better