I can't figure out a direct way to use the working pivot as the axis of a 'Symmetry' modifier but you can set the 'Working Pivot' as the object's pivot.
1. Turn on snapping and snap the object's pivot to the spot, vertex, face, etc., using 'Affect Pivot Only' and snap it to where you want it and it will become the pivot. Click out of 'Hierarchy' to set the pivot.
2. Then select 'Edit Working Pivot' and hit 'Shift+A' and click the object. The working pivot will snap to the place you put the object's pivot.
3. Finally, click 'Use Working Pivot.'
4. All 'Symmetry' and it will use that Working Pivot.
Rob Holmes

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