Will the Louvre give permission to who ever needs it to find out if the Holy Grail is in fact buried underneath the inverted pyramid?

The Da Vinci Code & the Holy Grail.

I'm guessing your question has everything to do with the Da Vinci Code book and movie. Although the novel contains elements of truth parts of it are fictional and, in fact, there is no proof that the Holy Grail even exists. Furthermore, nobody has any idea of what the Holy Grail is; there is speculation, yes, but nobody knows for certain. But if, however, they give permission, which I doubt, and in fact, there is something there it would be the discovery of the century; if not, you would have given in to the imagination of the author of the book.

Controversial criticism

There is a chance that Dan Brown would have seen the holy grail/sangreal documents in some form or the other. there is also a chance that the documents do exist and are buried under la pyramid inversee. However as mentioned before, there is none but a slim chance that the Louvre shall grant permission for further excavations to continue regarding this matter.

For those who do not know the facts: The Louvre got the exact location of documents as referred by Dan Brown earlier his in his book removed, so that visitors may not deface the property and try to reach the hidden chambers.