Ah you've discovered a new feature, only one octave is needed, key changes would be part of the game, like a star power, or cool power if you will. 4 perfect changes and Miles Davis comes out.

Actually Rock Band 3 shipped with support for keyboards both casual and pro-mode. You can actually get an adapter and plug in any midi compatible keyboard to your machine from a cheap mini-casio to a full size 88 key nord. The on screen chart only shows about an octave wide but will give you indicators on when to change.