Hi all,
here are my 2 cents from the 21st c.: I did some research on the matter in the meantime.
It might have been a former Anglosaxon custom to wall dead cats into a chimney since the dark Middle ages = Hard times for cats.
One source says that this had happened even in the Tower.

The Carlyle House Docent Dispatch of October 2005 says:
"Richard Sabin, Curator of Mammals for London’s
Natural History Museum, suggests that “it may have
been a common practice to bury cats in the foundations
of houses to ward off evil spirits ” Sabin goes on to say
that this practice “was a northern European tradition”
possibly brought “to England when the Saxons came
over or maybe earlier.”

Well, there is a specialist Richard Sabin at the museum, but he is not a cat specialist ( :-) ), he is specialized in in sea mammals. One should better ask a specialist in European Ethnology.  Of course, this existing superstition could have been brought to the former colonies. We crazy Europeans also exported the idea of witches.
Sorry, Richard: I did not found, within 20 minutes, any hint on a "Saxon" practise. Or "Northern European" practise.
But I found some hints that this custom was mostly spread in the UK.

I would also ask a biologist for the age of the mummy. :-)

And back to the museological question: Find the proof that this practise was intended  (By the owners or masons) as witchcraft. If there is no proof at least for the region or era, check the question: What does the mummified cat prove? A cat tragedy?
And WHY was the cat was walled in again?????

Cheers


Christian