Some way of iterating over both identifiers and content in a Polymer registry would be beneficial, like the vanilla Registry#getEntries method.

You can already do that, as it already implements Iterable interface, but on server there is only ItemGroup registry really. On client there are ones for Blocks/Items representations synced with clients

As far as I can see, you can iterate over identifiers or content but not both at the same time.

Ah then yeah