The castle is privately owned by an Italian-French company and has been closed for over 20 years as they await approval for an ambitious restructuring project for the castle. It is NOT open to the public, but I found the website for a local association that works with the property to organize occasional visits. Since this year marked the 200th anniversary of the birth of the marquis that built the villa, there have been several occasions this year when the villa was open as you can see here: http://www.sammezzano.org/iniziative.html

I see they also have a Facebook page - I'd suggest contacting them by email and through their FB page to see what they might be able to do in terms of a visit. Just know ahead of time that in August, lots of people go on vacation and lots of things close down. So don't expect too much in terms of organizing a visit so late and in August - they might not even checking email or the page at all right now. But I'm curious to know if you do get any sort of reply, so do share if that happens!!