Peter, you hate Italy and don't want to come anyway.
That's not true at all. See below.
If a foreign aircraft owned by a non Italian should be stuck in Italy due to mechanical problems it is highly unlikely that any tax would be required.
How can you be sure? They have chased me for months for bills I did pay, and each time a new person picked up the job I got chased again. To be fair, a lot of non Italian airports do this too (aviation suffers from crap admin all over the place - Bournemouth is a good start) but it illustrates the need for proper corporate transparency before flying somewhere.
In southern Europe, the rules are looser and a lot of stuff works by speaking the language and "having words" with the right people, and a lot of people like that. It's great to get away from the rule-bound north. But when flying internationally and when one cannot speak a word of the local language, one is relying heavily on proper procedured being followed. The last thing one wants is a law on the books of the destination country which will pile on a few 000 tax after X days' parking - unless one is able to arrange some "local contacts" (which often means bribery).
Anyway, little GA like me is irrelevant. What Italy will notice is the people with the £10M boats and the £10M jets not going there anymore. The only question is how long this will take. How long did Sardinia have its "luxury tax"?