I always teach my students to use what I call a 'Multi Fill'.
When it comes to that fill in moment and you're on Var B and want to stay there: Press, for example, in fairly quick succession, D A C B. Next time, try C D A B. It well works and even my 10 year old students who have S9xx or PSR3000 keyboards all use it as a matter of routine.
What I asked for at meetings with Yamaha requires a new style engine - Each Variation has four sub-variations, based on major, minor, 7th and diminished/augmented chords, and each of those has four sub-sub variations, automatically selected by the keyboard on a bar-by-bar or two bar-by-two bar basis, so each style also has sixteen possible Fill Ins, determined by the chord and the up/down variation movement but randomly chosen within that by the keyboard if you follow my drift.