This code from the docs:

BsonClassMap.RegisterClassMap<MyClass>(cm => {
    cm.AutoMap();
    cm.SetIdMember(cm.GetMemberMap(c => c.SomeProperty));
});

...fails if SomeProperty is on a base of MyClass.

Based on another issues I read (CSHARP-396) I thought this might work:

BsonClassMap.RegisterClassMap<BaseOfMyClass>(cm => {
    cm.SetIdMember(x.GetMemberMap(c => c.SomeProperty));
});

But that fails too. Both cases fail with cryptic message "Invalid memberMap."

The only workaround for mapping via code seems to be remove inheritance.