Readonly might lead to corruption or unexplained crashes. Depending on your constraints one option would be to replace Poser with your own script. Your script would then fork/spawn Poser in child wait mode (your script waits until child process finishes) and then your script kicks in and merges your changes into LibraryPrefs.xml. Simple file i/o could be used for this and avoid any firewall issues possible with a rpc type solution.