Transferable MediaStreamTrack: Within Agent Cluster or between Agent Clusters?

[alvestrand]

https://w3c.github.io/mediacapture-extensions/#transferable-mediastreamtrack does not specify any restrictions on transfer of a MediaStreamTrack.

When investigating the possibility of implementing this extension, we found that there were fundamental differences in the complexity of transferring within an Agent Cluster (basically same-origin iframes and DedicatedWorker) and transferring outside an Agent Cluster.

It's fairly obvious that there are potential use cases for cross-cluster transfer, but it's also obvious that supporting them is costly.

Should we restrict transfer of MediaStreamTracks to within an Agent Cluster, or should we require universal transferability?