THAT would be an interesting General Order to find, indeed :o)
The Duke of York didn't hold the post of Commander-in-Chief 1809-1811, so...oops!
In any event, I've not encountered evidence for, nor reference to, such a thing. I have reviewed the "General Regulations and Orders for the Army" issued by the Adjutant General's Office, Horse-Guards, 12th August, 1811 (and supplemented through 1815) and there is no hint of such a General Order from so anyone at the top.
Best wishes - Howie
P.S. Although I gather that an order from the Horse Guards in 1801 has loosened up the strict requirements of the "Rules and Regulations" (from Piers Mackesy's book on the Egyptian campaign). Notwithstanding the Regs and Orders, successful practice stretched the loopholes permitting it to hallowed custom.