Here is how this can be accomplished:
The Administrator needs to go to the Workflow Design tool in Mashup Composer and click on the Submit Transition and go to field overrides and set the Field you want to be in Read Only to Read Only.
Then go to the Mashups Admin Tool Click on the Project that controls things, for example if you have a project and sub project, if the project controls the subproject click on the project other wise go to the sub project and click on it. Then Select the Default Fields tab and select the field you want to require and select edit button, then select attributes and click override and then make it a required field. Then it should work.
If this makes no sense send me a email and I can call you.