At the end I've decided to try to solve my problem by writing a plugin.
When called it will for each book in the library
1) rename all authors and book title by adding a "special" temporary string at the end
2) rename again authors and title by transforming in upper case and removing the special string
3) check if the path length from the Calibre library is excessively long and eventually rename the title by cutting the last part
This approach, if I'm correct, should:
a) avoid problems with the alternate use of linux and windows (at the cost of having all authors and titles displayed in upper case; cost which is not a matter to me)
b) permit me to avoid any modification to calibre source code (which then I will have to maintain for future releases)
Is this approach viable? Any suggestion?
PS I've thought that to avoid unuseful renaming for books which have already been "upper cased", perhaps I can first off all compare the upper cased author name and title with the actual path on disk and proceed only if the two strings do not agree. If they agree I can directly pass to the following book.