Redmine

I want to have a field for new issues within a project and don't want to use the issue id which is incremented per entry. I want to have an auto-generated GUID. Has anyone successfully figured out how to create a custom field that does something like this? I know it's not built into the default custom field types - but perhaps someone has modified to the code to do this. http://www.redmine.org/projects/redmine/wiki/RedmineCustomFields