I was also interested in what brand(s) they were using. But I was not able to see any markings on the keyboards, even in the video close-ups that were featured on the large overhead screens. He has his boards arranged three high in two banks (opposite each other). Each bank has a black backboard which covers the rear of the boards where you normally would find the brand label.

All the boards appeared to have 88 keys, however. On one number near the end of the show, Yanni clearly was using the pitch bend wheel on one of the boards to achieve the sound he desired. For the most part, he played them straight, however.
Paul C.