Bvt Major Trailrider,
Actually, since you are "detailed" to the Ordnance Corps, you would typically wear the uniform and rank of your regular corps, ie a major of Cavalry. Assuming that you were detailed after 1872, you would not wear the brevet rank, only your substantive rank.
However, since I realize you want to wear the uniform of an ordnance officer, you want to wear plain dark blue trousers (same color and fabric as the coat). Field officer coats were double-breasted.
The uniform regulations for 1872-1884 are provided below. Off-hand, I don't believe this changed for staff officers in 1884 (though I could be wrong).
For General Officers, Officers of the General Staff, and Staff Corps: Dark blue cloth, plain, without stripe, welt, or cord.
For all Regimental Officers of Cavalry, Artillery, and Infantry: Light blue cloth, same shade of color as prescribed for enlisted men, with strip one and one-half inches wide, welted on edges; color, that of facings of their respective arms, except infantry, which will be dark blue.
US Scout
Bvt Brig Gen