You should search around for IIS settings that can be used.
Of course, if IFrame module allowed to point to a file saved to a DB, you would be able to do that without any issues...
Also, you don't need to allow your content editors Admin rights - just add a role (i.e. "Content Editor") and give that role Edit rights on Text/HTML module. Then, users with that role will be able to edit content of that and ONLY that module.