Found a couple of mentions of staff-breaking by a Steward (Worcester) in Shakespeare (Richard II)
Quote:
GREEN
We have: whereupon the Earl of Worcester
Hath broke his staff, resign'd his stewardship,
And all the household servants fled with him
To Bolingbroke.
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&
Quote:
HENRY PERCY
No, my good Lord; he hath forsook the court,
Broken his staff of office and dispersed
The household of the king.
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For what it's worth.... Certainly it seems that Tolkien is tapping a primary world tradition in having Denethor break his staff when he surrenders his office.