Seems to me that you can do that just changing the related project's workflow. Pay attention to the permissions, restrictions and project roles.
For further details, the following link could be helpful:
In addition to what Paulo already stated, modifying the underlying workflow by adding postfunction "assign to XY", you can easily modify the transition screens in adding (only) the assignee field. Result would then be Greenhopper asking simply for an input of the assignee during specific transitions. Not quite an auto-assignment, but still quite flexible and lean.
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