As part of making the Offline Internet Archive available on OLIP as well as IIAB (and Rachel and standalone), we've implemented the opensearch spec. Though having problems testing on OLIP currently.

Would it make sense for IIAB to implement opensearch as well ?

It looks to me like there is a UI (Search box) that composes a bunch of queries that go to plugins on apps like interntarchive, the UI then collects those results to return a common response.

opensearch wasnt hard to implement from the plugin side (the plugin never has to parse XML). It might be a bit harder from the common server as it probably has to parse XML and then turn into HTML.

I'm not sure if there are any competitors to OpenSearch (since I think Google supports Opensearch). Obviously XML sucks (technical term) but I havent seen a JSON equivalent. It would be trivial, from the app side, to return a semantically equivalent JSON structure if that made the implementation of the common part easier.