This is tricky. Unfortunately, the flexibility you gain by building an
IFrame-based application is offset by the lack of OpenSocial
integration. Since the browser's security policy prevents two pages
running on separate domains from accessing each other's DOM directly,
I don't know a clever way of accomplishing this. Technically, the
gadgets library includes an RPC mechanism, but this is slow and not
well documented since its built for container developers, not
individual applications.
In general, we discourage app developers from building apps in this
fashion because it limits what their apps are able to do. This is one
example. Another is the difficulty in getting friend data into the
frame's source due to limitations on URL length. If activity posting
is a must, I suggest retooling your application to run outside of an
IFrame or, alternatively, waiting for the REST API, which is nearing.
- Jason