For me (on Linux) it works as I decribed. I use the same method to update version numbers in furniture library property files (a furniture library .sh3f file is also a zip file).

You can first try to rename the .sh3d to .zip. Maybe that solves the readonly problem if you are sure the file is not readonly. Maybe it's in a directory where you can't edit?

If that doesn't work then you can extract the whole file in a directory. Update and save the Home.xml file. Select all files in the directory and create a new zip file. Change the extension from zip to .sh3d and you're done.