Hi.
As for my problemi I proceeded this way.
First I created 2 different locations: "Italy" and "Italy Islands".
Then I assigned the different zones to each location.
Then in the weight based shipping extensions I associated two different pricing for the 2 locations above.
Now when the buyer chooses a zone located in an italian island an overprice is applied.
It works. I know Your problems is more complicated, hope this can help.
Alessandro