Given that there is no way to make Site editable by the requestor, what we would like to do is have an Additional Field that will be common to all Catalog items called Site Update to allow them to change the Site value (since it is not directly editable).  When our trainer was here we suggested an additional field and he said it should work but we never worked it out before he left.  

Here is out thinking:

-We will populate the 'Site Update' field with an identical list to the Site field.

-We create a Custom Trigger that looks for the 'Site Update' field to indicate they want to change the Site

-We create a script or call a class that sets Site = 'Site Update'

We have worked out the first 2 items, we can't find anything to get us started on Scripts or Classes so any help with this last part would be immensely helpful.