It is possible to visit the second floor of the Starbucks above the Shibuya Scramble in a wheelchair. You simply need to use the Tsutaya video rental shop’s elevator and go to the second floor. However, because you are borrowing that store’s elevator, you must wait until they open at 10:00am. So, don’t go too early!
The ordering counter is on the 1st floor, so you may need staff support, or a friend, when ordering.