"highly exclusive" is a correct and usable phrase in written English.
You can use it to refer to something that is available or accessible to very few people, in comparison with the general population. For example, "This luxury resort is highly exclusive and only the wealthiest clientele can afford to stay there."
Beyond the exclusivity of its content, Les Mannequins is also highly exclusive in quantity: the book on view is one of an edition of 37. Informally known as Into the Night Life, the official title of this 1947 Henry Miller book is actually a 67-word run-on sentence.
At Cambridge, he belonged to a highly exclusive, secretive essay and discussion society called the Apostles.
The Harvard encampment, much like the university itself, is highly exclusive.
Despite those numbers, it is a highly exclusive affair: not everyone can be part of the show.
Given the size of the deficit, this auction represents just drops in the bucket — highly exclusive drops.
The study group is a highly exclusive group to be a part of with just 19 participants fielded from a group of over 150 qualified scientists and engineers.
There can be only 24 OMs at any one time, making it highly-exclusive.
The upside is clear: highly coveted, exclusive looks that can be ordered directly upon collection presentation.
A longtime high-fashion favorite, Stone debuts on the list with earnings of $4.4 million after striking deals with venerable houses like Versace, Prada and a highly coveted exclusive contract with Calvin Klein.
"ModCloth's catalog offers thousands of choices in clothing and accessories, including highly differentiated, exclusive styles and an extended range of sizes that appeal to a broad demographic of women," the Walmart statement read.
Their system is highly concentrated, exclusive and works well for the artists who make it into the inner circle.
