Not all browsers display browser busy indicators for loading content in an iframe: it’s more to the seller. If you load a page in an iframe, you cannot suppress indicators in browsers that do this, and don't turn them on in browsers that don't.
In chrome, even creating a completely empty iframe ('') still briefly triggers a busy indicator, so I don't think you can cheat using srcdoc or iframe.contentDocument. You can get away with loading most of the iframe content with ajax (which will not lead to its further promotion). I don’t think you could make it 100% quiet.