changing the iFrame src in the image properties window

Hi there, I currently have a bit of a problem. I need to add some dynamic (server side) functionality to the Image Properties (fck_image.html) window. I'm using the FCK in DotNetNuke and all I want to do is set some of the textboxes to readonly depending on the user's role. I've changed the src of the iFrame to fck_image.ascx and I've added some functionality - all of which is working outside the iFrame - however when I try to open the image properties window the iFrame hangs and the loading image just repeats. Now I've checked that the JS / iFrame is pointing to my intended file and that there are no code errors in my new fck_image.ascx. It just won't work. I've also tried changing the ASCX to an ASPX file - still no success - any ideas anyone?

Thanks

MS