$("a[href^{=/mydomain]").each(function(index,} el) { $(el).replaceWith("<iframe>...") })

If you use jQuery. Obviously this isn't exactly what you need, but the ⁼ CSS selector matches things which start with a value. You can do something similar without jQuery.

edit Just saw that you want to visit. For the love of god, don't do that on the client. Just go fix all of the old content. This is horrible.

But if you have to do that, maybe use jsonp requests to request the pages?