Submitting a form to an hidden iframe isn't a good idea, i remember trying it with another project and i ran into a big bug with Safari and some version of MacOS. This browser will not submit a form to an iframe unless you manually visit the target website first. I remember many people had reported this bug and i had to download MacOS and test it under a virtual machine to confirm it. I searched everywhere but never found a solution except asking users to change their default security settings.
So i just realised i will run again into same problem if i do that. I really need to get AJAX cross-domain submit to work.