My legs are giving me a headache.

[Newton Hunt]

Of course such things happen at the wrong time.  Murphy is.

Steinway makes those legs, by special order mostly.

Gluing the leg up can be done but keep in mind the stresses that a leg
is put under when the piano is moved.  Very likely the leg will
rebreak at the same point because the integrity of the wood has been
compromised.  

I hate to suggest this but you might consider taking a leg off a B or
other D not is current use or in a no stress location like a teaching
studio.  Use a tall stool to prop up the piano so it can be used.

Call Michael Mohr at the factory and tell him your problem.  Usually
he is able to help more quickly than the parts department.  He knows
what is on the floor, parts only  knows what is in their inventory.

Lots of luck.