We should be able to override the Assignable User permission in the permission scheme by using workflow properties, jira.permission.assignable.*. The property can either be applied in workflow statuses or transition, and we can limit the list of the Assignee based on project role by using the following property:

• jira.permission.assignable.projectrole

Though the property works as intended in most of workflow transitions or statuses, apparently it's not working as expected if we apply the property in the Create Issue transition. Things to note here is that the property will not work when using the auto-complete Assignee field, however if we switch the field to use the old style 'Dropdown', then the property works as intended.

Some sample screenshot illustrating the behavior:

• The Assignee list is displaying all potential assignable user, although we have applied the workflow property to only list down the project role of the ID 10100:
  
• But if we use the drop down list by setting the frother.assignee.field = false, we get the expected results:
  

Only 2 users is expected to be listed in the Assignee list after applying the workflow property

Tested on JIRA 5.1.7