I also have same requirement. I'm using a custom report with node details and software revision which shows the PanOS version as of now.
To display in the summary page, i have added a custom Universal Device poller with SNMP id obtained from internet.
Lets wait for some expert to help us.
I did build a custom Palo Alto node details poller a while ago that returned the software version, if you go into Manage Pollers in settings, and then Thwack community pollers you should be able to find it to test.
I can't guarantee it works with the latest versions as we no longer have any Palo devices.