With Ketron arrangers, they've divided this, so your endings and drum fills are literally different pedal buttons. On the BK, they've kinda lumped it in to two steps as you say.
99% of the time, I simply accept the variation that I want to end, and just hit the ENDING button on my pedal. The sounds are progressive, 1 is quieter and 4 being the most "complex". I usually work up that chain in the song and end in the most dynamic manner.
OK since I have the BK sitting here in front of my at my mixer, I can do a fast experiment.
I've never bothered to even see if they even can do the trick like with the intro (before starting the beat, be in variation 1, hit INTRO, select the intro you want, ie: 3, and when you hit start, it does intro #3 and moves in to variation 1). I just basically tried it now, and it doesn't seem to work in the same way, so you CANNOT "preset" your ending in advance, BUT...
The only way that I got it to work for your particular situation, (and it CAN be done, but it takes TIMING), is possible.
If you note, whenever you hit the ENDING,
it completes the current measure THEN inserts the ending.
If you are playing a song with 4 beats per measure, on the 2nd or 3rd beat hit ENDING, then select the variation you want BEFORE the measure is over, it completes the measure and uses the ending of your choice... et voila, you can be playing in 1 variation and end in another, BUT if you start the process on the 3rd measure, miss that "window of opportunity", it ends with whatever ending that your current variation is set to.
Takes some pedal dexterity, but is VERY doable.