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This may be  a compromise solution, which could also be
useful in other contexts: What if the object passed
is derived from dict - presumably that doesn't help
because the point is to do low-level calls to the
actual dict's lookup functions.
 Now, suppose we modify the basic dict type, so
that before throwing
a KeyError, it checks to see if it is really a derived
object with a method __keyerror__, and if so, calls
that and returns its result (or lets it throw)?
Now you can make objects that look like dicts, and
act like them at the low level, but can automatically
populate themselves when non-existent keys are requested.
Of course, __keyerror__('x') does not have to make
an 'x' entry in the dict; it could make no change, or
add several entries, depending on the desired semantics
regarding future lookups. It could be set up so that
every lookup fails and is forwarded by __keyerror__ to
the __getitem__ of another object, for instance.

The cost of this to the 'normal' dict lookup is that
the need to do PyDict_CheckExact() each time
a lookup fails.