Window.postMessage() has a targetOrigin parameter that can be set to a URI (to ensure the message reaches only a specific url). It can also be set to * of course (not recommended), but is there a way to specify multiple URIs as allowed?
At present I'm simply firing off one postMessage() call for each domain, but this seems a bit hacky to say the least.
Unfortunately you can't. You should either provide "*" or single specified domain.