Premise: When specifying the url type in a Google Gadget, the remote server's content is displayed directly inside the iframe. As a result, attempting to retrieve friend information or similar data via Ajax requires cross-domain requests, turning it into server-to-server communication via a proxy—which is useless without a RESTful API.

Apparently, Orkut does not allow Content Type="url".

MYSQL database connection using PHP for my gadget ? – Orkut Developer Forum | Google グループ

Specifying Content Type="url" seems to return a 404. While returning a 404 itself is reportedly a bug, I was able to confirm that even if Content Type="url" were working, accessing OpenSocial across domains still requires RESTful API access via a proxy. I have not confirmed whether this is just an interim measure until the RESTful API is officially introduced, but it seems my initial premise was not mistaken.