The yamaha model which featured this solo mode actually played a bunch of pre programmed solo riffs. I think it was not used after this model because it always played the same solo based on the key you were in and it got too busy within the accomp for most people.
AJ would have to explain this better but my take on the SD5 was that it just added more flavor to the actual style and doesn't try to solo over it but rather add some extra tracks and get a little more excitment out of the style. It was interesting to note that by just playing with different velocities on the keys, you could have more tracks play within the styles, sometimes adding horn stabs or Organ parts, etc. Again, only a few of the styles AJ demo'd had this riff feature. This was a prototype keyboard and the units when they ship will have lots of these "riff" styles pre loaded.
George Kaye
Kaye's Music Scene
Reseda, California