Answering my own question: I went to my Miles and Smiles account and chose "claim missing miles" (under "Mile"). Entered the details of my two canceled flights, and the miles were immediately credited to my account! Unbelievable.

I must say I have boarding cards for these two segments; I don't know whether this would have worked had the flights been canceled a few days earlier. But it seems that if one is checked in for a flight that is subsequently canceled, one can claim the miles immediately.

Shuly