Use Flash to allow scrolling using the mouse pointer. It could also have a semi-transparent dot as the cursor.

By passing requests through a custom proxy server, requests to external sites could pass the iPhones user-agent string.

Allow the address bar to be toggled so you can test a site on the larger view point in Mobile Safari

Create a button that allows other sites to link to a dedicated test page.

iPhone does not have a visible scrollbar. This can be achieved with simple css on the iFrame