There are three "assignment" types of Post functions out of box: assign to current user, lead developer and reporter. There is no option to assign to Project Lead.
I also tried to add a Proj Lead custom field hoping we can update the field using Update Issue Field post function. However, it seems only the system fields show up as the available fields for update.
Any plugin out there that would allow an issue to be auto-assigned to Project Lead during a workflow transition?