What were the Concorde's emergency procedures for an uncommanded droop-nose extension in flight?
As commented, reducing speed (and replanning flight) is all there is to it. From the Concorde abnormal procedures:



The nose has three positions:


Up
5° down
12.5° down


Holding the nose at 12.5° requires hydraulic pressure. If hydraulic pressure is lost, the crew can manually release the two uplocks, and the nose (and visor) would fall and stop at the 5° position – two additional jack uplocks stop the motion at 5°.