hmmm.. As far as the design goes the failure of a nose gear to downlock was extremely,
EXTREMELY remote. There were three ways o lowering the nose leg (Normal, Stanby,as well as free-fall). As a matter of interest the main gear sort of had four ways, where the free fall could be assisted by bleeding engine P3 air into the equation also.
Being rearward lowering, the airstream of course helps matters a lot with respect to the nose leg lowering
One of the wing'd chappies I'm sure can come up with the flight procedure for such an event. (Never happened in the 35 years of Concorde flight testing and airline operation).
Dude