Mega voices are designed to be played by the Styles, or a DAW, with modified velocities. Trying to accurately play the mega voices by hand is almost impossible, because velocity will increase loudness, then at some point, trigger a new sample, then again adjusting volume.
I don't think it would be hard to for Yamaha to write some code, that would allow a slider, to move between the different samples, and ignore the volume changes of the sample This would make the mega voice, able to be played live..
I have recorded hand playing mega voices, then fixing velocities, to get a very usuable result. It would be an extra feature for the Genos.