Addition of a LAST-MODIFIED property on all VTIMEZONE objects. The value should correspond to a timestamp of the execution.
Note that the TZID usually includes the date the VTIMEZONE was generated, so you might not need a LAST-MODIFIED.
If you still want one, it should be a command-line option, and vzic-merge.pl would need to be updated to ignore changes in the LAST-MODIFIED property.
You should also check that Outlook will accept it. Outlook is pretty fussy with iCalendar files.
I was thinking I would need it to check for new definitions in an auto-update feature I was implementing. Turns out it isn't necessary as I just take the latest definition anyway.