You asked for "any" legal reference. Here's one, albeit local in nature but very plain.
http://www.dot.wisconsin.gov/travel/...-standards.doc
scroll down, item 9
The quote:
A displaced threshold is a runway threshold located at a point other than the physical end of the runway. The portion of the runway so displaced may be used for takeoff but not for landing. Landing aircraft may use the displaced area on the opposite end for roll out
And onther....
From the FAA, Central Region - Airports Division, AIP Guide Index, Runway Commissioning, Data elements:
The link:
http://www.faa.gov/arp/ace/aip/aip-1121.cfm
Scroll down to item 51.
The quote:
The portion of pavement behind a displaced threshold may be available for takeoffs either
direction and landings from the opposite direction. This length is included in the total length of the
runway.