passing in javascript values into iframe tag

The only way you can pass values directly to an iframe of a page with a different domain is through the url, as querystring values, anchors, or perhaps some form of rewriting. Even if it's on the same domain, that's probably the easiest and safest way to pass it.

Main document:

document.getElementById('IframeID').src = "somepage.html?seed=" + custom_seed;

Inner document:

var seed = window.location.search.substring(window.location.search.indexOf('seed=') + 5);
if (seed.indexOf('&') >= 0) {
    seed = seed.substring(0, seed.indexOf('&'));
}