How can I call a javascript function from within iframe?

I've had a read about and I'm aware of the fact the same-origin policy would prevent the DOM of an iframe accessing the parent window's DOM and vice versa if on different domains.

However, I do, in this case, have at least some control of the parent DOM too (it is still on a different domain). I'm developing a site to sit in the iframe but will also be asking users to add some javascript to the head of their page.

Given that, is there anyway I can expose a javascript function in the parent with that included javascript?