Perhaps my understanding is incorrect, but I was under the impression this was only permissible aboard a US Navy vessel?
Public Law 829 (77th Congress) Sec. 4(a) :
"The flag should never be displayed with the union down
SAVE AS A SIGNAL OF DIRE DISTRESS"
&
Title 36 U.S. Code §176(a)*
(a) The flag should never be displayed with the union down, except
as a signal of dire distress in instances of extreme danger to life or property.