How can I customize the "Assign Issue" operation screen?

I am playing around with the new Screens features. Kudos on a powerful and easy to use tool to streamline data entry.
I have successfully created tabs and added the ability to edit (custom) fields when a developer resolves an issue.
However, I have not been able to figure out how to to the same on the "Assign Issue" operation. That is not a workflow operation, and in the "Add Issue Operation To Screen Association" form, the dropdown " Issue Operation:" doesn't allow for "Assign". Am I missing something, or is it just not possible to customize this screen in the same way?