eivissa:
I cannot help you with the legalities of such actions with respect to JAA or Spanish law.
What I can do is offer my perspective as a fire pilot, albeit on the left side of the big pond. This issue comes up frequently in California, and here are my thoughts/comments in no particular order:
1. The aircraft would need to be co-ordinated with the "official" effort.
2. Helicopters are generally used to support other efforts---unless it is purely a grass fire then water drops alone will not save property.
3. The pilots of the helicopters need to have an understanding of fire, and fire suppression techniques.
4. What happens if the they hire helicopters and the homes still burn up.
5. Some homes cannot be saved due various factors--defensible space being the primary factor.
6. More helicopters is not necessarily better.
In reality the best way for them to protect their homes would be to get "firewise"...there is lots of information here:
About Firewise