@lowpro
BDRB has the possibility to add avisynth scripts. Assuming your source is 1920x1080, try
Code:
crop(0,120,-0,-120) #1.78 to 2.40
addborders(0,120,0,120) #letterbox (mask) for for blu-ray compliance
(It works in avisynth, I didn't try to include it in BDRB)