The Beluga is too small in length, width, cargo weight capacity and volume to be used for 787 Global Logistics.  The LCF is nearly 1/3rd larger in volume than the Beluga.   ref the graphic below:
 
 The 747 LCF is a good deal larger than any of the world's current and future cargo aircraft.  by comparison
747-400 LCF at 65,000 ft3
AN225 at 38,500 ft3
A380 Freighter at 40,000 ft3
A300-600ST at 49,000 ft3  
MiG