When mounting an ISO as "read only" it causes various errors when performing tasks in the gui.
An error occurs when attempting to add a user with a home directory that is pointing to the read only iso mount
An error occurs when you point the web-server "Document root" to a read only file-system such as an ISO mount. This occurs even if you do not check "enable directory listing"
I am assuming pointing anything at a read-only file-system will cause errors with various services in freenas. I can only verify this is happening with the services I listed above.
Possibly having an option of "read only" in various services would prevent freenas from trying to manipulate the read-only file-system.
I am running 0.7 Khasadar (revision 4919)