Google Gadgets (on the iGoogle custom homepage) used a similar scheme for exposing a select API to untrusted JS running in frames. I was mostly exposed to it be

Google Gadgets (on the iGoogle custom homepage) used a similar scheme for exposing a select API to untrusted JS running in frames. I was mostly exposed to it because Mapplets used the same backend to allow user widgets on Google Maps. The proxy Maps API objects (used to spawn map markers and so on) were similar in API to their non-proxy counterparts, though obviously every interaction with the other-frame content became asynchronous, and that led to a bunch of interesting small differences that could trip you up if you didn't appreciate the reality of all your data being copied back and forth across that boundary.