Great that it worked for you. I read about mkimage / cpio some time ago, but didn't remember. Here's the link:
https://www.mobileread.com/forums/sho...0&postcount=13
BTW - there are also means to extract the contents of official sony firmware updates by using shell functions included on the reader:
https://www.mobileread.com/forums/sho...d.php?t=166871
http://projects.mobileread.com/reade...an/PRST1/tools