iframe without an src attribute

Not sure if all browsers support "about:blank", so I'd just go with your own blank page then.

Another idea: Why not add the whole iframe using javascript instead of just the src?

Standard approach when creating an "empty" iframe (as an iframe shim, for example), is to set the src as javascript:false;. This is the method used by most of the JavaScript libraries that create iframe shims for you (e.g. YUI's Overlay).