Here is a brief chronological rundown of General Halleck's military career activities following Grant's appointment as Commander-in-Chief, US Army.
12 Mar. '64 to 19 Apr. '65
Chief of Staff, General HQ, based in Washington DC. (He was also a pall-bearer at Lincoln's funeral).
19 Apr. '65 to 27 Jun. '65
Commanded Military Division of the James, based at Richmond, VA.
27 Jun. '65 to 1 Jun. '69
Commanded Military Division of the Pacific, based in San Francisco, CA. (During summer, '68, he visited Alaska, and the department of Alaska was added to this military division on 18 Mar. '68).
9 Jan. '72
He died at Louisville, KY.
Appears Halleck got appointed to senior administrative command roles in various military divisions (with each division comprising multiple departments), for the next four years after the war.
But his activities during the final couple of years of his life are unknown (to me).