Photobeams are going to be your best option.
There are many to choose from but
here is an example. If you open the installation manual on the link you will get an excellent idea of what you are in for as far as the installation.
They are quite expensive to install because of the buried cable and power supply needed but you did use the word "mansion" in describing the customer's house so that shouldn't be a problem.
ADI has a decent selection also.
